I don't know if I think it was premature, the position is quite lost. Only in a team game would I continue such a miserable position.
I saw that you play the Caro-Kann and unfortunately for me I have always had problems against this opening. I studied up before the game and decided to go with this version that is relatively new to me, especially with 5... g6 which I had not seen before.
After playing move 15 I was still in theory and thought the position couldn't be too bad for me. Black's 15th move was not mentioned in any of my books so I thought I was okay and tried to figure out why it was not a theory move. But the more I thought about it the more it seemed to be a very good move indeed. I just couldn't figure out what to do.
Now, it didn't help that this was during the period where I was the most sick with Streptococcus but I am certainly not blaming that for my loss. Still, I thought about the position for a few minutes here and there when I felt up to it and couldn't figure out what to do. Eventually I only had a few hours to do the move so I did. Turned out it was a horible one from which I could not recover.
Looking at the position now, I still cannot figure out what I am supposed to do. Return the Knight back to c3 perhaps. Even the computer seems to be somewhat uncertain suggesting a whole bunch of moves with about the same evaluation. (The tactic Nxa7 would have been nice had it worked out but unfortunately it does not.) Anyway, overlooking a one move combo in a correspondance game is not one of my finest moments.
I don't know what else to say... the game would of course continue with Qc2, Rxb2, Qd3, Bh6, Rb1 and it's just lost. Nothing I can do but end the misery of looking at the darn thing.