I'm going to try to get my girlfriend to see a doctor
Why do you think she needs to see a doctor?
It is because she has been thinking negatively for a while
Your girlfriend is a negative person and it is nice of you to be aware of her health, but no doctor can really help her. Unless she gets some therapy with medication I have learned that pessimists are always pessimists. You will be always dealing with it from her so be comforting and encouraging to her.
She thinks I'm a no good piece of worm and will never leave me.
I'm sorry she's treating you that way, but I'm sure she's just in a bad place right now. All you can do is continue to be there with her and help her out in any way you can.