5. ... Bc5. I don't think there is anything particularly wrong with this move. According to the database at chessgames.com, the most popular choices in this position are, Bb4 (87%), d6 (5.4%), Bc5 (4%) and a few others.
6. ... d5 is just a tactical error. If you practice tactical problems, you will eliminate these errors from your games. You want to drill simple 1, 2 and some 3 move tactics that only take you a few seconds to a minute. This is like learning the fundamentals of a sport.
I know I made a lot of mistakes in this game, but I'm especially interested in figuring out how I messed up the opening so bad. The outlook was pretty bleak after losing the knight on move 7, so I'm wondering how early I really made a mistake... Also, I feel like I could have fought back even though I was down a knight. Is there any basis for that, or is it just wishful thinking? And I clearly screwed up in the last couple of moves, so what should I have done differently? Or should I have resigned earlier? Thanks.