Confused dude,
I really think that it's not your obligation to cheer up your girlfriend. You are very nice to her, you're very caring and that you're not doing anything wrong to make her feel this way. I have a feeling that she has depression. Please encourage her to see a doctor so he/she will prescribe her antidepressants. No matter what you'll do, her depression won't go away.
It's great that you're going to school to improve yourself. Don't ever feel guilty that you could not see her as often as you like, nowadays, one has to have the adequate and proper education to land a good-paying job, now with a lot of unemployed people. Don't waste your emotion of being guilty, you haven't done her wrong.
If she won't go to a doctor to help her with her depression, her situation will get worse. You don't want to be with someone who has en emotional handicap and try to nurse her. You need to be with someone who is healthy mentally, physically and emotionally so you'll have a happy life together.
If you want to have a future full of problems, then staying with her is what you should do.
Take care and I hope you'll choose your woman properly.
