I hope that others can help chime in too, because I don't have a great answer-only my own anecdotal experience.
When I was 20 I was still learning how depression was affecting me and to be perfectly honest, I didn't know that it was depression. I just figured it was normal and it was just part of being human to be sucked into a dark place and held there, timid and scared and powerless. I know now that that is not true, but I also think that nothing someone could have told me would have been able to really teach me that.
I say that because she might not being willing to recognize yet in herself that that is a burden she carries. Some patterns in life help alleviate it: exercise, eating well, having good people in your life, etc but learning to deal with it is tough.
If you are seriously interested in her, then taking a bit of time to understand how mind of a depressed person works might be helpful for you so you can understand why she may do or act in certain ways sometimes without being angry.
Probably telling her that you understand that sometimes everything can sometimes feel overwhelming and you understand if she feels that way but that you would still like to just chat with her, with no expectation or stress, if she needs a "break" like that so you still know she is there might help. By letting her know that you aren't holding her failures against her, she will be more able to share her failures with you and be more honest. When she is more honest with you then you can see her as she is, and understand why she does things or is unable to do certain things, instead of her having to make up excuses to cover her tracks. If you can try and be compassionate and understanding, that will probably come in time. You can decide along that path if this relationship is indeed worth the effort, and that is fine, but at least you will be judging it on her truths and not a protective facade.
In general, being compassionate and trying to understand the other persons perspective and asking them why they did a certain thing is helpful for mature relationships. One of the best pieces of advice I have ever gotten about relationships is a way to have that conversation. By telling someone "You did x...and I don't like it", it sounds accusatory and they will get defensive. But if you simply reverse the order and use the word "I feel", as in "It makes me feel y, when you do x" then it is much less accusatory and will stimulate them to discuss why they did something because they care about how you feel. You can also say things like "How can I (or we) try and fix this to make this <problem A> better?", even if its their fault, by you taking responsibility for their problem they will rush to try and help fix it because they know it is not your fault.
Best of luck, and good on your for being eager to learn and grow as a person. Compassion and kindness makes life so much easier and wonderful. =)