How do you tell a friend that you don't depend on them because they haven't been dependable historically?

In a constructive way? Because I have forgiven this person, but that doesn't mean I want to set both of us for failure again by expecting as much as she says she has to offer again. Also, I need her to be a legitimately decent listener before she is sad that I didn't talk to her about stressful things. But that's a different issue most likely.