If you're truly having a "long-term slump" then it's entirely possible, even likely, that you are a long-term losing player. Probably not what you want to hear, but it needs to be said. All too often people are happy thinking that their wins are purely down to their maginficent poker skills and their losses are down to the evil luck monster, but this is not always true.
Anyway, some tips.
- Increase your study : playing ratio. By studying more (a) you will tend to learn more than playing with a lack of confidence and (b) you will gain more confidence that the moves you are making are actually the right moves.
- Remember poker is a long-run game. I dunno how long your "long-term slump" has lasted but any sample less than 100k hands is pretty meaningless.
- You don't "fold rags and they hit". (a) You only remember the times they do hit which leads to skewed recall and you thinking it happens far more often than it does, and (b) as above poker is a long-run game and doing stuff like limping with 94o UTG because the last two times you had 94 the flop came 994 is a long-run losing proposition.
- Take a break. If all else fails, just sever from poker for a while. You can either continue to study or just take as long as you think you need completely offfrom poker, as you prefer
- Play a different game. If you play MTTs, try cash. If you play NLHE, try PLO. Obviously if you play say $50NL and have never played PLO before I don't recommend you jump into $50PLO, but move down a bit and try a new game. Strangely, the less you know about a game type, the less stressful playing it tends to be and the more there is to learn about a game, the more enjoyable the learning process tends to be.