What to do about a pessimistic girlfriend?

I've been seeing a girl for about 3 years, and she's in the process of getting her degree, living with her parents and seeing me. She wants everything to happen overnight. She wants her degree, wants to be married and all that. One day she tells me she isn't ready to get married yet, but wants to know why we never go look at rings or talk about wedding plans. The other day she says to me, I don't think you want to get married, I don't think we are on the same page.

My question is, what am I suppose to do? She is always down on herself about trivial things, she changes her mind constantly. I'm getting to the point where I'm sick of it, I do everything for her, support her in everything, I pay for everything, what more does she want from me?
What to do about a pessimistic girlfriend?
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