18...Bb7 "I think Black has a significant positional plus" 

Yeah, I much prefer your position.

21...b5 I'm not worried about white's kingside build-up. I don't believe he has the resources for an attack there. As long as I own f5 he's not getting in.

It's great how you constantly dominate White, preventing the freeing advances d5 and f5. White can only activate his bishop by giving up a pawn! 

25...bxc3 - This is a mistake. I am actually opening diagonals for my opponent's bishop! 

Interesting observations. 25...a4 seems stronger as you say.

You missed 27.f5!? an opportunity to free his bishop... and more, due to the unfortunate alignment of your rooks and queen.

On 29. Nf6+ I was seeing the ghost of Kf8 Nh7# if my knight had still been on e7. I also missed the pin of the f7 pawn after Qh7+.

When a higher-rated player sacrifices a piece, they are telling us that they feel they'll get compensation. Maybe they're wrong and it's our time to move up and their time to move down... but I think alarm bells should be ringing in our heads to really, really check that taking the piece improves our position.