Worrying inconsistency in playing strength I recently found time to make a study plan and improve my playing strength. Using this plan, I was able to achieve a 1900 average rating on online chess sites. 

Since a couple of days, I have in a certain sense "lost the feel" of the game, for seemingly no reason at all, and my playing strength quickly dropped to the 1600-1700 range. No matter how hard I try, I don't seem to regain those abilities I had mere weeks ago. 

How do I get that "chess focus" back? Has anyone had this experience before?
Don't worry you're not alone in this, I can assure you almost all chess players/enthusiasts go through such periods, not only at the start, but throughout their whole chess experience. Of course there are many elements involved here, that at the end of the day define your performance, but let's stick to those that are most common for all: (bear in mind these points are purely speculative as I cannot possibly know what may be truly causing your performance drop without actually knowing you!)


Learning curve: Basically whatever new we learn and try to incorporate with our chess knowledge, be it new tactics, new positional ideas, more opening varieties, weird endgame situations, etc. there's some minimum necessary time involved (different for everyone of course) to be able to put these new ideas into practical usecases and actually make them work. Why? well one immediate reason is that everyhting in chess requires some amount of experience until it comes closer to being understood. Prime example is when we all first learnt about different checkmating ideas, and then wanted to immediately apply them in every game we played, and of course instead we just end up losing because by giving such strong importance just to some checkmating pattern we become completely ignorant to the more basic ideas underlying each position (activity, safety, structure etc.). So I think you most probably had an immediate boost to your rating (reaching 1900) more by blind chance than anything else.  
What you should retain from my first bullet point is: stay positive (i.e. care more about your play than rating, it's just a number after all...) and keep playing your usual chess, and whatever new you learn along the way, as you keep playing you will get more and more appropriate chances to putting them into good use, meaning you should let the occasions present themselves first and then try applying what you've learnt. By no means you should take this statement as equivalent to "not controlling the evolution of the game", after all that's the whole point of positional play(keeping a grip on the position and imposing its evolution on your opponent to the extent that you can), my point only concerned specific precise new things that you may learn, e.g. rook endings, or known theoretical drawn configurations etc. In this way you will slow by slow incorporate everything new that you learn much more naturally. Simply put by playing more you will discover more about the applicability of the things you've learnt.
Now one orthogonal reason to the previous two would be: ratings are always fluctuating, specially on online servers (we most often just play for fun and mess around hence may end up losing a lot of points suddenly-also bearing in mind that they're usually short time control games->hard to keep a consistent rating), so I think everyone experiences this at any level, what is more important is your average rating, say in the last 2 months, that says a lot more about your performance. I myself have always had drops of roughly -+200 rating points every now and then, same goes for most of my friends and people I play against. Chess is really a lot about struggling, because of the extremely rich dynamics of the game, in every game there are so many things that are completely different and unknown compared to our previous experience.
Another point you brought up was "chess focus": One's mind may very well be completely at ease before sitting down and playing a game of chess, but may still lose his/her focus easily during the game if not careful. At any stage of the game, it is extremely crucial to stay completely alert of at least the most important elements of the position (no need for examples here really). But how? well apart from the fact that everyone has different ways of getting hooked into a game (concentrating), there are general ways of staying focused: e.g. whether it's your turn to play or not, you should be continuously calculating relevant lines to your postions, and absolutely avoid saying to yourself "I'll just wait and see what he/she plays then I'll think about my move", instead you should always assume that there are more things to the position at hand that you see at first sight, so the more you try to calculate different variations (within the time you have) regardless of whether they're working lines, you will nonetheless discover more about the space of possibilities hidden in the position, and always assume your opponent is going to play the best move you can think of. All of this, as part of your general attitude towards chess-playing, will to some extent improve your focusing abilities. 
Enough with speculations :) My last advice: just keep up your playing routines and don't allow for big gaps of inactivity in your playing as much as you can, don't be ignorant towards your losses, instead dig into them as much as you can and you'll have a lot more concrete improvements resulting from that than anything else. Hope this has been helpful!