It sounds like you are a bit lonely. Either that or you grew up with the expectation to keep up appearances around people so you suppress your emotions until you get overwhelmed when alone. I often felt like you described during college and often in the years after. I think most people worry about not being good enough or not having done enough. A good day's work often makes me feel better for a while.

You mentioned auditions. Are you studying music or theater? Performance arts do tend to be stressful and emotional. My guess is that you probably look more accomplished than you think and your peers probably look at you and worry that they aren't good enough too.

It's good that you know your triggers and try to figure out why you feel depressed. I find that once you let someone see that side of you, its harder to pretend to be normal around them. I was a mess during college too and I was surprised to hear someone say I was popular. You are probably too critical of yourself especially when you are alone without anyone to compare with. When I'm alone too much, I read about people like Albert Einstein and feel bad that I'm not as good as they were. Then when I'm around people, I realize I'm doing ok compared to those around me.

I think people must think highly of you and you just don't know it because they are also hiding their self-doubt. If you really do have clinical depression, you could talk to the school counselor just in case. Its nothing to be ashamed of.