I don't think therapists should make promises that are not realistic and I think it is cowardly if they do - a cop out. Sure they can technically promise not to abandon you since it is unethical to do so. The real issue here concerns the definition of abandonment by ethical standards versus a client's standards - I think it's clear that the two are quite different and that's why clients are often so hurt. Regardless of whether an "inner child" is asking for reassurance or not, it's just wrong (and stupid, I think) for a T to make such promises. Promises made to a child by a parent cannot be replicated by other people - those promises are made in the context of unconditional love, something only a parent has. No matter what a T says, they are not that person and they cannot make those promises.

OP I do give your T credit for admitting her screw up- I think that's a rarity and she probably really is remorseful about it. But she is right and it is just something that needs to be accepted, as awful as it makes you feel.