you're too hard on yourself! This is a really cool rendition of Isaac. The thing about fanart is that you don't have to accurately draw the character as they are portrayed in their material. I think this is great!
Damn, were gonna suggest you to draw him, and then I found out you already had drawn a nice one I like that you have the holographic ammo count on the gun too, nice detail ^^ But yeah, the design is pretty complex, I want to draw him, but my drawing skills are worth nothing ლ(ಠ益ಠლ)
I have to say that while if you were going for a 'perfect' drawing of Isaac, then yes you did fail. However I love this piece, I feel the way you rendered Isaac suits the mood of the game and story better.