analysis, please. A classic sicilian encounter

This was a game that I felt pretty happy with, but would appreciate advice and analysis over. I am a fairly weak player(1150), but feel that I am getting better over time and that I am finally fairly strong tactically. This is a game with a much stronger player, and I will give you my thoughts and reasoning for as many moves as necessary, starting at move twelve( this is also an exercise in annotation for me, which hopefully helps my analysis and overall chess skills in the first place) . Any help is much appreciated...

I think you played pretty well.  here are just a few things to think about.

when you developed your dark squared bishop, you lost a pawn and allowed for a nasty foothold for whites two knights.  maybe something a little more aggressive, ie pinning your opponents knight to the king or trying to take over the center.

second your bishop sacrafice was risky.  it opened up a file for your opponents rook directly to your king, as you soon found out.

it seems you like to play aggresively and did well when your opponent went out of mainline play.  good job at exposing your opponents weakness - keeping his king in the center.

I'm too lazy to give you a full depth analysis but there are two moves that stand out in your opening.  Your 8th move and your twelfth move were both clear blunders.

8. ...Bd6 is an error I've made in the past myself where you block the path of your queen to the pawn that it was guarding.  Now black wins a pawn.

12.  f3 Bxf3.   You mentioned here that you thought you were going to lose a piece anyways but that isn't true.  Just move your bishop to safety.  There are plenty of safe squares to get that bishop to.  As it stands if after 13. gxf3 your opponent had done something like Bxf6 followed by Kf2 or O-O after your recapture he'd be out of your pin and up a piece, and your King side would be just as screwed up as his.   

After that though, my casual assessment is that you did a good job exploiting the pin that your opponent allowed.  But as I said, those two particular moves, especially move 12 stand out as blunders.

5...e5 immediately would have been correct. As it was, your opponent developed his army too fast and overran you.

cheers everyone.

Happyfanatic; if I moved my bishop to a safe square on move 12, there is still the matter of

13 bxf6

with a gain of tempo, or

13 Nxf6 gxf6

14 bxf6

with a messed up kingside and a huge advantage in tempo and initiative. I'm sure any chess engine would agree with you, however.