If you're committed to saving/improving her, you can pick up some books on dealing iwth low self-esteem, even if she doesn't acknowledge the issue, you can perhaps help her by acquiring greater knowledge. You can also consult a therapist yourself and ask how they suggest handling someone who's resistant to the idea of therapy, or just tips on dealing with her LSE. CEBT is the most effective therapy for this sorta thing, last I heard. You can read about it here:
Cognitive behavioral therapy - Wikipedia, the free encyclopedia
You can pick up some books on this sorta thing. That said, you want to be careful about imposing too much on her, people with LSE usually feel a sense of helplessness, so you might wanna try to help her gain some greater control over her life.
You may also want to consider something my gf of 2 years and I committed to: we are not going to stick together because we've been together for a long time or because we've had fun times in the past. The relationship has to be good now for us to stick to it. It's basically zero-based thinking: would you enter into this relatoinship if you were given the choice to today knowing everything you know now? If not, then get out. This isn't easy, its emotionally difficult, but its not complicated. You just might not be compatible anymore and its time to move on.