*this* is your problem! You should check to see if the square you're moving to is safe before you move there! 9.Nd2 and you're fine. The easiest way to improve your chess ability is to get into the habit of looking just two moves ahead -your move, your opponent's move, your replay to that, you're opponent's reply to that. If you can do that consistently, even if you learn nothing at all about "theory" or openings or endgames, you'll be able to play at the 1400-1500 level.

Agreed: this was very careless. Had you been looking just a little further ahead, you might have considered playing 8.h3 (instead of 8.Be3) to prevent the pawn advance. But there was nothing wrong with the bishop move. Attacked by the pawn, the knight would have quite happy to have gone to d2, with quite reasonable prospects.

10.Nb5 is better, since then black can't play ...Rc8 or ...O-O-O as his a-pawn hangs.