I've been depressed, so I know what this is like. Not being depressed is a
state of mind. Bad things and good things will happen in anyone's life; it's a certainty. You've got to remember to cherish and rever the good things while brushing off the bad things and not letting them get to you.
I recommend checking your internal dialog. You know all those things you say to yourself in your head while life happens on the outside? You may not even notice you're doing it. Pay attention to it, it's important. When you realize your inner voice has said something negative, like, "God that was stupid of me. I'm such an idiot!" -- check yourself. Reevaluate that thought. You may not even remember that internal dialogue the next day, but it <i>does</i> affect you in profound ways.