After 11... dxc3, you could've just taken the pawn, since the c3 knight blocked the queen's view of d3. From there, you would then aim to place your knight on the holes/outposts on d6 and f6, perhaps via Nb5 or (if the d-pawn ever advances) Ne4.
At 21. g4, there may be an improvement. g4 prompted the knight to move to d4, where it was looking to occupy f3 and serve as a general pain. We can see that you do have the advantage of two pawns, but meanwhile black has finished his development and is looking to take advantage of your weak, somewhat open king position. An improvement could probably come from Bd2, protecting your hanging rook, or even Qc5, tempting an exchange of queens for simplification so that you can make use of your two pawn advantage in the endgame.
That exchange at the end following 26. Rc8+ wasn't in your favor, obviously, since it allowed black's rook to eventually make use of the open c file. Taking with the bishop would've still been followed up with Rxc2, so there's really no improvement you could've made after that 26. Rc8+ move. As you suggested, 26. a4 would've been a better move to make.
I played this game in the Winower Poisoned Pawn Variation of the French Defense. After leaving the opening theory, it was hard to find moves that didn't immediately lose- it felt like my position was just slowly being constricted. Any advice on how to improve?