missing a big opportunity

I had a game today against a play with a rating of 2096 (I am 1705), I had a great position untill I fell into a simple fork in the end and lost immedietly..

before I'll go to the game: recently I have a problem with long games, at the end of the game I'm losing to a simple tactic (like simple forks, skewres, etc.), which are much below my level, and it didn't heppened only in this game, but also in some other games. any idea of how dealing with such a problem? (note it only started heppening a month ago or so, before I usually didn't miss those pesky little tactics).

and to the game (Analaysis by me, note the variation of 12.b4!! I saw it in the game and almost played it, but was to afraid as i coudlen't see it to the part which I have enough compansation, I saw it only to about move 16):

you are a lame version of forkman