What were my mistakes?

Hey I just played this game and I was wondering if you could help me analyze it? I have added the thoughts I had after I finished the game, didn´t use a computer. What were my mistakes? Who do you think was the better side in the opening after 10. Qc2 Nd7? I would appreciate any thoughts about where I could have improved my game, and how I could annotate my games better.

Neither Black nor (I claim) White can hang on to the gambited c-pawn in a double d-pawn opening without losing. Therefore Black erred by playing 7...b5? This is when White had an advantage and should have played the standard attacking move to shatter Black's pawn structure for that error--8. a4--but instead White played passively with 8. Be2. After that, White was probably unable to ever regain that pawn, so was permanently down a pawn.

You noted some of the more obvious, embarrassing errors: Black's tempo loss with 19...Qh4 20. g3, and Black overlooking a backward capture 37...Qd3+ 38. Qxd3. In fact, I'm going to add this game to my collection of games where players overlooked backward captures, since I don't believe I've never seen that type of mistake mentioned in any chess book, so I'm collecting such games for people (like myself!) who made that error, so they won't feel so alone.

I'm not really sure where you lost the game. probably you should have been more aggressive, maybe you've let him play too much giving him the time to find a good place for all his pieces, on the other hand you seemed to me, but it's only my impression you really didn't have a clear plan on what to do, queenside or king side? maybe this is what made the difference. beside of that I think f4 it's really dubious, and maybe you've understimated the potential of his LSB bishop. I think until 16. ...f5 you were equal but personally I would have preferred to play white here. 4 moves later after 20 g3 you're already bad. I don't think the main problem is to drive black queen away it's rather getting rid of his most active pieces, so it is his LSB and his knight. if you could have done that you would have more likely entered in a raw endgame with rooks and queens, his doubled extra pawn shouldn't be enough to make the difference.

Thanks for the replies! I will go trough the game a couple of times with your  thoughts in mind, appreciate it!

Semi-slav defense, (and most queen's gambit declined lines) I play generally feature a bishop relative pin on knight. Nf6 should be played first prior to the bishop pin rather than going immediately after the queen. If you waited for him to play Nf6 and then Bg5 then you can play pawn to e4 then b5 then pawn to e5 and then h6 and then Bh4 and then g5 and then Nxg5 h6xg5 (and white reclaims a pawn with damage to black king-side pawn structure).

As for the soundness of retaining the pawn--I think black can get away with it but he generally has to rely on white compromising his own position.