Thinking and doing. Right now, you're doing too much doing, ie playing chess, and probably not enough thinking, ie study. The risk in doing that is that although you are absorbing chess, your improvement has hit a brick wall as without the requisite study to compliment all the games, you are not getting all you can out of the games, as you're probably not recognising where you can improve, imo.
I think you just need to rebalance your chess, and examine what you feel are the weak points in your game that are costing you. For all of us, there are usually more than one, so it helps to prioritise. Study in these areas, and be conscious of them when you are playing.
Keep on playing, by the way, just dial it back to a point at which you feel you are improving again. Studying your games is an invaluable tool, and seeing where you went wrong, and learning from where you did.
Likewise, use the different forms of chess you play, in different ways to improve your chess. Use blitz to focus on honing your instincts and tactics perhaps, as well as a sounding board for openings or lines you want to understand more, as there will be no quicker way than a good few games of blitz to throw up all the issues in the lines you are looking at.
Correspondence can be a place to study things more deeply, where you have the time and the freedom to think a lot before you make your moves. The games should be of higher quality too, as more time means less likelihood of basic mistakes, so you are likely to have deeper battles to learn from.
Between correspondence and blitz, your over the board play should improve, I think, and before long you should have broken out of the stagnation you feel.
Mainly though, seek to identify the weaknesses in your game. These will always be your guide on where you need to focus your efforts to improve.