I often feel the same way about my boyfriend -- that he would be much better off without me and dating someone without all the emotional baggage. Every time I say this, he says to me "Stop being silly; I'm not going anywhere. This is my decision, not yours." My advice would be to tell her how you're feeling, but don't push her away unless she's already on her way out.

Sorry you're hurting so much
Best wishes,
Ro