what r my weakness

i m black in this game......i lost this can u plz help to find out my mistakes and my weakness

what r my weakness

on which area i should work on

you played a good game. I suggest you'd better look after your pawn structure.
like doubled pawns, and isolated pawns.
Also, taking more initiative might have helped you here, pinning for example.

ps. 30.Qe1+ could have captured his passed pawn. 

First thing to work on: stop giving away pawns.

I dont think this is a right place for endless discussions. white could have easily mated earlier (37. Qg8+ etc) , so I guess he wasn't in a rush either. there may be something to learn about checkmating

Wow, I totally thought you were white... so I'm deleting my post.  Why don't people just post the board from their point of view?

Spelling

11... Bd6
Unlike other pieces, do not try to directly confront Knights, unless it is forcing
a retreat.

13... Be7
I don't know if you tried to protect your queen or something,
but this move was unneeded. Your pawn can easily counter.
I myself would've move your rook to e8

25...Qe4
Checking the king with your queen would've save your bishop.

Agree! :)

Only resign if your opponent is over 1600.  Anyone under that level is capable of stalemate, so don't give up.

I think by playing both ...Nf6 and ...Nc6 gave your opponent targets. Usually Black plays one or the other move, not both, at least in the games I've seen. Wouldn't you like to be the one pressuring his position with your bishops, and castling quickly to exploit the open e-file? 4...Bb4 5.Bd2 Nf6 6.Nf3 O-O looks like an aggressive approach giving your opponent room to go wrong. But I've only played the French a few times.

You missed a tactical win. 25...Qe1+ 26.Kg2 Qe4+. You've got to explore all CHECKS and CAPTURES if you want to become a stronger player.

After 36.b8=Q you are a queen down... a good point to resign. If you haven't learned yet how a player wins with K+Q vs K, check out this tutorial.