How do I learn to understand the middlegame? I am trying to train tactics as well as my general understanding of the beginning/middlegame. I spent a good bit of time understanding endgames, per chessmaster recommendations and those I can understand.

As far as tactics, I have been able to "train" them through chesstempo.com and "see" lots of variations through chesstactics.org. 

But how do I learn to understand what I am training? I feel like I can execute tactics now without thinking, but I am not really sure what I just did. It just "feels right" because I trained it.

So what I am really asking:


  Is there a good place to get a real understanding of the middle game, or is it just raw calculation? Maybe I am missing a positional understanding?


I am thinking about getting a chess.com membership and going through the chess mentor courses. 

I feel like by just doing raw training I am missing out on the real soul of the game.
In principle, the middlegame is indeed just raw calculation. In principle, the entire game of chess boils down to only that. But since the space of possible move sequences is so vast, chess is of course too complex from the standpoint of pure calculation for that to be all that we do when we play. And after all, even our computational superiors (chess engines) need to be guided with built-in positional understanding in order to play a good game of chess.

So yes, if you're feeling fairly comfortable with your grasp of calculation and the execution of tactical operations, but often find yourself "not really sure what [you] just did," then it does sound like positional understanding is what you need to accrue. A good way to look at things is this: tactics are a means to an end - and at bottom they are the means to all ends in chess - but it's not terribly useful to see tactics as the ends themselves, and that is where heuristic positional knowledge is most useful, as a guide telling you what purposes various tactics should be put toward.

For instance, former champion Vladimir Kramnik described the play of two of his predecessors thus:


  To a certain extent, Smyslov was the pioneer of this style, which was later brilliantly developed by Karpov, i.e. the gradual mounting of positional pressure based on the most accurate calculation of short lines.


Now, regardless of whether you want to play like a Smyslov or Karpov, or more like an Alekhine or Tal, I point to Kramnik's words here just because they bring out this idea of letting positional considerations guide your hand, while your tactical acumen gives you the wherewithal to achieve your aims. OK, so how to acquire positional understanding?

Here there are many many options. You mention chess.com's chess mentor course; I have no knowledge of that, and so have no opinion. I can point you to a couple of books that are designed to begin to steer amateurs toward an expert/master understanding of positional play:


How to Reassess Your Chess by Jeremy Silman. This is perhaps the most widely-known book on the subject these days, and with good reason I think. Silman does a fine job of getting the reader to (1) look at positions through the lens of various types of imbalances - e.g. who has more space, is one side's bishop better than the other's, is one king safer than the other's, etc. - and (2) begin to use those imbalances as guiding factors in determining what your plan/goals should be - e.g. I have two bishops against two knights, so I'd like to open up the position to make use of the bishops' long-range power.
The Giants of Strategy by Neil McDonald. This book is less systematically instructional than Silman's, but it is still instructive. McDonald looks at examples of play from Kramnik, Karpov, Petrosian, Capablanca and Nimzowitsch in chapters grouped around positional themes (e.g. the seventh rank, outposts, pawn breaks, planning). This isn't an overly advanced book, and is probably well-suited to the same audience as Silman's. I wouldn't recommend this book alone for gaining positional understanding, but I think it can act very nicely as a supplement to Silman, say.