where was the mistake?

I believe I had control of this game and after reviewing it I believe that the mistake was in 18.c3...f5, I should have played ...Bb6, do you agree? Could you tell me what you think of it, or where could I have done better? thanks a lot. Oh by the way I play black.

I'll appreciate all your comments, thanks.  

5. ... b6. Your light square bishop already has a nice diagonal to come out on.

10. ... b5.  You are shutting in your bishop. This is like playing a piece down. 

11. ... Nb6. Where does the knight have to go from b6? 

18. ... f5. Yes, this is a mistake. You expose your king to a lot of danger. You don't have the defensive resources on that side of the board because your light squared bishop is stuck on a6. 

I agree....While you had lots of space on the queenside your pieces weren't really doing anything active at move 12.  After you castle I think white even missed a more appropriate reply with n-f5.  With possible sacks coming and just a superior position

at move 24. your opponent had game won then but didn't know you could have move rook to f6 and cut-off that attack