My ex-fiance (I'm happy every time I write that down, thank heavens we did not get hitched) did not have low self-esteem in general, but she did have trouble handling life when it got hard and in certain areas she did not believe in herself. When she was happy none of it mattered, but as soon as anything went wrong it was, "Oh, I'll never be able to afford a house here, let's move, my job sucks, they won't promote me, I don't want to work for someone else, nag nag nag nag nag you didn't do this and that, kyphan!"
It went away once, but when it came back I knew this would be the ebb and flow of her life. She was always going to be like this.
Run away. Run away as fast as you can. They will bring you down. Find someone else, you cannot, will not, and shall never "save" a low self-esteem woman or a woman who does not believe in herself.