On move 19? I couldn't really see any forcing tactic. My first instinct was to play Qg5 to try for some cheap tactics involving Bxh3.
lOOKS GOOD , but white seems to have enough to defend against tactical play at this point thus the search for a strategical Plan. Because white will certainly challenge the Rook on the 7th with (Rb1) and my backward queen-pawn attacked by his queen-bishop don't help matters.
So you want a positional move then. In that case, a5 followed by Rb4. That fixes the weak white a-pawn in place, and cements your control of the only open file.
That was my choice also but it seems the position calls For :
Black wants to open the "F" file with the pin at (f2) being the focal point of the black rooks. Also threatens (..f4) as in the game, If a4xf5..Bxf5 then Bxa3 ? ...Rb6 loses material . So after a4xf5..Bxf5 (BxB..RxB) black has an attack and the iniative due to the attack on the isolated Queen-pawn.
Should be Decisive ( No Engines Please ) Black's Move.
Or Suggest a Better Plan