Is there any good app for learning openings? My great fail is that I cannot learn opening or initial play. 

Normally I only play when I can connect to my mobile app (Chess Free on iOS), and play at a very low level (adjusted as a single bar), and repeat that level until I have some domain of it. Unlike a human player or a friend, that sort of games cannot teach me which are the errors I am repeating consistently (some adaptive app?). 

I've tried puzzles, but they are so artificial, I can't learn how to start with them. 

I normally can defeat some people in apps such as chess.com, but very fast I reach a level superior than my real one, when I do not defeat anybody, and I don't get how to improve. After that the app download me again into trivial opponents, and the same repeats over and over.

So I don't increase my level, and I've been stuck in that from years, without any advance when facing real players.

Which I would like is to freeze the difficult level, and play let's say 100 times with players at my very exact, and me being the person to decide when it is time to move onto better players, let's say by raising 50 points only?.

Or equivalently, is there some app playing with strong focus on a specific selectable opening, let's say for example, the Dutch defense, and the game could go in turn of such opening? Making me able to play 100 times an specific opening, before increasing the level of such opening and/or before changing such opening?
In terms of apps, other than the good suggestions made so far, I definitely recommend most of "Chess King" apps, and specifically for openings:


Chess Opening Lab (1400-2000)
Chess Opening Blunders


These apps have a theory section, where various types of openings are discussed (closed, open, etc) and a practice section where you get to solve small problems specific to each opening (which you will be able to choose from). 

On another note, from your description I gather your difficulties may not be necessarily opening related, but a tad more fundamental. So what you can always consider, specially at a beginner's level, is to learn basic principles about opening up the game, and not necessarily memorizing specific known opening lines. What this entails usually can be roughly summarized with the following points:


Deciding on your own style of play: open positions (e.g., 1.e4), closed position (such as 1.d4, more positional style) or the various other semi-open/closed ones. 
Piece development: whatever strategy you have in mind, you first need units to execute it with, so prioritize piece development at the beginning, get out the minor pieces, occupy the centre, ...
King safety: the cliche is that you should always castle as early as possible, which is not a bad advice, but in general what matters is that you consider your king safety, and not naively undermine its importance. 
Create useful space: Big part of development phase is to occupy parts of the board (often the centre) that are most accessible to your pieces (specially the minor pieces). For example, if you ignore occupying the centre of the board with your central pawns, you will have a lot of difficulties to establish centralized knights. This is often a very difficult aspect of the opening phase, as it is non-trivial and different for each opening. But think simple, you want to have active knights and bishops, so always ask yourself during your play: what structure do I need in order to post my knight here or there, or in order to have this or that diagonal open for my bishop.  
Improve piece coordination: Your pieces (specially minor ones at the start) need to speak to each other, and opening theories in short provide optimized recipes to this end while refuting/limiting your opponent's play (but this is another discussion). Meaning, you have to think in terms of your collective pieces if you want to execute some plan or strategy. Amateur players tend to just opt for small tactical plans that they execute with 1 or 2 pieces: e.g., a queen and a bishop, a knight and a bishop (e.g., a bishop of f4 and a knight on b5, a typical attack on c7). Such small size attacks are often easy to parry once spotted, so try to involve as many pieces as you can for your attacks, this will make it harder and harder for your opponent to hold off. You may ask: but so when are the aforementioned "small" types of attacks useful: well, think about them in this way: on the one hand, these are small maneuvers that allow you to potentially perturb your opponent's desired setup and on the other hand, they can be seen as forceful means of rerouting your pieces to better posts. Forceful here suggests that the rerouting is without loss of tempo. 
On the more practical side of things, and more related to the first point on style, you have to give different structures sufficient trials until you fully understand an opening to its nitty gritty details, details that may only at a higher level become relevant, but the good thing is the better you understand a line, the easier it will be for you to adapt if your opponent tries to refute something or deviate abruptly.
So pick up a theme of a opening you like, and play it as much as you can until you master it, then try something else, and the same routine again. This way, very soon you will have a rough idea of what to play vs many different openings, e.g. you will have a basic system vs e4, d4, and ... Again there's no recipe for what I referred to as "your basic system", because what it really means is that you will know in what terms to assess different openings and on your own out of the principles you pick up along the way, you will naturally come up with good responses. 
Let me give two basic examples: Catalan (c4, d4, g3) players really like their bishops, hence the double fianchetto that often results from this line (idea being to preserve the bishops and reduce opponent choices of exchanging them), but to actually know how to leverage the two bishops takes a lot of experience and positional understanding in order to know e.g., how and when to break the centre, do I play Rc1 before c4, do I take back with knight or pawn on d4 if they take ..., and a great deal of tact, as otherwise a good opponent will easily find ways to force trade of bishops. Another opening like the Dutch: primary emphasis of the Dutch is to control the light squares on d5, e4 and the latter without having to play d5, so you preserve the possibility of playing d6 if you so desire, or if the position demands for it. With this in mind, white wants to counter these very reasons, and regain better grip over central d5,e4 squares, so first thing white annoyingly and smartly does is to give up the dark square bishop for the strong knight on f6 (which is a good re-enforcer of d5,e4 squares), e.g., 1.d4 f5 2.c4 Nf6 3.Nc3 e6 4.Bg5 Be7 5.Bxf6...  
Pawn structure: its importance cannot be emphasized enough: pawn moves are the most committal moves in chess, mainly because pawns move irreversibly. This means that you need to make sure to what pawn structure you want to commit and not naively reduce your choices of expansion or defense. That's why the English opening is so powerful: it's one way of being conservative about your pawn structure, in order to have more options in reaction to what your opponent goes for. Other example, in sharp Sicilian positions, an experienced player on the black side knows not to impulsively move either the h,g and f pawns impulsively to fend off white's push, instead you have to wait until you really have to play h6,h5, g6 and so on (these are very tempting defensive moves when white is coming at you with all they have, which is the case in sharp Sicilian lines). The sooner you commit, say to g6, the earlier you've created an unamenable target weakness for your opponent to aim for. To showcase, here's a beautiful game by Kasparov (black) vs Adams:




[Title "Michael Adams vs Garry Kasparov - 2005"]
[fen "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.Be3 e6 7.Be2 Qc7 8.Qd2 b5 9.a3 Bb7 10.f3 Nc6 11.O-O-O b4 12.axb4 Nxb4 13.g4 Be7 14.g5 Nd7 15.h4 Nc5 16.Kb1 Rb8 17.h5 O-O 18.g6 Bf6 19.Rdg1 Ba8 20.Bg5 Be5 21.gxh7+ Kxh7 22.Nb3 Nxc2 23.Nxc5 Na3+ 24.Ka2 Qxc5 25.Na4 Nc2 26.Kb1 Qa3 0-1


Notice Kasparov's king-side pawns until the end ;)

Anyways, the list goes on of course and by no means should you take the above points as complete, but hopefully they do give you some pointers and I have to stop at some point :) 
But what is important for you to take away, is the ways you can think about the opening phase without ever having to sit down and memorise lines, instead, from now on whenever you see a new opening being played, try to look at it with the above points in mind.