It's just a struggle until move 20...
At that point, your king is in bad shape because:
1) Your K-side light squares are horribly weakened, and your opponent has an active bishop set to go to work on them
2) Your opponent's queen is sitting comfortably in a very active place, ready to enter the battle
3) Your opponent's rook completely dominates the wide open c-file
4) Your king is stuck in the center
Obviously, after 20.Bxh6, your king is under attack. Your opponent is removing a k-side defender, and you must recognize this. To stave off the oncoming attack, your next move HAS to address one or more of those four problem spots, or you're a goner.
A quick engine check gives 20...Qe5, offering the trade of queens, and not giving white time to blow you up with Bh5.
A look at alternatives suggests Bc6 would shut down the c-file for the time being, giving you a additional flight squares, and chance to fight on.
g6 gives your opponent new and different targets on the k-side, but effectively shuts down the light squares over there for at least the moment, giving you time to rearrange your defense.
Qf7 does the same.
None of these moves fixes your position, or gives you the initiative, but when your situation is dire, logical moves that address problems (versus immediate, unthinking moves that do not) are the difference between putting on a tourniquet and bleeding to death.