How can a >2000 player stop making basic errors? For the last 3 months or so I've been unable to concentrate and play chess to the level that I am used to. I'm rated 2050.

I've been suffering from the following symptoms:


Being completely unable to see the board in it's entirety. I've done silly things like put my queen en prise because I just didn't see that an enemy rook is on the same file
Being distracted whilst playing. I have this odd habit of working hard to gain a winning position, then my mind drifts to some music that I like. Inevitably I throw my advantage away
Playing 'hope chess', i.e. I may analyse just one variation a few moves deep and conclude it is good for me. Of course I miss my opponent's best move


My question is, how can I get out of the rut?
From the way the question is written it seems like these flaws in your chess are uncharacteristic, and that you recently noticed that they've suddenly become way more apparent than before. The first point about putting pieces en prise does indeed seem uncharacteristic for any player >2000 Elo, unless we're dealing with very short time controls. 

For a beginner, putting a piece or two en prise during a game is to be expected, but for someone of expert strenght it's indicating that something is going on. Most likely it has to do with momentary lapses of concentration, and it is very difficult in general to know why these happen. Has something happened in your life recently that puts you under a lot of stress? Then that is very likely the culprit affecting this area of your game, and you should work on improving your personal life in order to get rid of these annoying mistakes.

The second part, about getting winning positions and then not being able to convert them, is not uncharacteristic for a ~2050 Elo player in my opinion. Strong players tend to be remarkably resilient, and they will not yield an inch unless you force them to. Keeping this in mind, it becomes clear that it is very dangerous to think of your position as "winning". That way of thinking makes it much more likely that during a game you will start relaxing too early thinking the position will win itself, and consequently letting your opponent off the hook. Instead, I believe it is much more appropriate to always keep in mind that your position is nothing without you! Without you there to accurately improve it, your position will most certainly deteriorate, no matter how good it is to begin with. Don't forget this; do not relax until the game is over!

The part about "hope chess" should be taken care of if you remember that your opponent is not supposed to be your friend during the game. If they can avoid it, they will never actively cooperate with your plans. However, if the hope chess has increased in the last couple of months it could be caused by the same thing that is causing you to drop pieces. Stress has a way of making people mentally exhausted, and this will inevitably lead to chessplayers 'letting go' the moment they think they see a solution to their problems, unwilling to strain themselves mentally even further by double-checking to make sure the plan is air tight. In this case the solution to the issue should be to work on improving your personal life to eliminate mental exhaustion as a factor in your game.