Why did I lose this?

We both made mistakes...but I was pretty excited when I took his rook and evened the game.  Then...I slipped.  What could I have done to save my endgame?

I believe you should have tried to keep the bishop pair. 2 Bs vs B+N was a lot better for you than for him; so maybe you could have avoided the black bishop exchange.

Moreover.. after move 41.b4, you had a decent position but you screwed it up with kd7? (no offense). You had no real need to run for his passed pawns, since they can't get pass your bishop without some moves. You had a nice pawn wall in the kingside that you should have pushed.  40. ....; f4 was a good alternative. by your move 39...a5, it even looked like you were preparing that push but somehow forgot about it.

sorry for not adding specific lines, but I'm kinda sleepy and would probably post wrong ones.

cheers

33 .....Be8 seems like the beginning of the end. Why not 33. ....a4 ?

At least put a fight up for your rook pawn. Obviously when you gave up that pawn its a white pawn storm on the queen side, creating threats you can not handle.

If you had of protected it with your bishop you could have attacked the white Knight with your king.

Also your moves from 38 onward were terrible. You have no plan and you dont seem to care for you pawns. You needed to try to force a passer on the king side.

At least something like 39 ...h4 would have at least put up a fight. But the game actually ended at 33 when you gave up your ciritcal a pawn.

At move 9. you should have played Nxf3 winning a piece after white recaptures

You didn't protect your pawns at all!

Opening mistakes.  Move 9 and 15 missing a piece and giving away a piece.