My fiancee has suicidal thoughts and I don't know how to continue, what should I do?

I have been in a long-term relationship for a couple of years. Before we got in a relationship she had been slim and gained a fair amount of weight before I met her and we started dating, she goes to the gym, and from the beginning she said she was trying to lose weight, its been two years and she hasn't really lost any. I am still attracted to her but I am sure that the weight does not help with her thoughts.

She has never outright threatened to commit suicide if I leave but will make comments like "if I wasn't with you I wouldn't want to be here" and things of that nature which leaves me feeling trapped. She is also very negative, at times it seems like I am in relationship with Eeyore from winnie-the-pooh. She says she's getting better but sometimes it doesn't feel like it.

I love her so much but the suicidal thoughts and negativity are really starting to weigh on me. I want to stick with her and help her, but I don't think I could do this for the rest of my life if nothing changes. Does anyone have any experience with this or any advice from people that maybe dealt with similar issues or thoughts? Thank you for responding if you do.

Updates
1 mo
She's not always negative, there are good times. There are so many positive things about her, and I don't want the positive to be overshadowed by the negative. We are long-distance which makes things harder, but it's just when the times are bad they are really bad.
My fiancee has suicidal thoughts and I don't know how to continue, what should I do?
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