My therapist is definitely filling that good enough mother role, and I feel very lucky to be in a therapeutic relationship with her, though it's challenging to be so vulnerable. I think a therapist can definitely be good enough: I did have a really good therapeutic relationship in high school too, though brief, so, I guess that gave me a sense of optimism even though I had subsequently very bad experiences in therapy for a while.

Hard to say if your expectations are unrealistic without knowing what they are. "Good enough" is all about consistency- the mother-figure responding in a consistent, caring way, but not about being perfectly attuned, since it is through misattunement and delays that the child-figure eventually experiences some of its growth. I think the nature of the good enough mother is that she will make mistakes, perhaps including lying, but that she is devoted and humble. I would ask myself "what were my therapist's intentions" when I evaluated a mistake she made- was she trying to betray or lie to you, or trying to help you and did it badly? Was it a breach she worked to fix, or one she let fester? I try to judge a lot by intentions: I may be *very* upset by missteps in therapy, where I feel my most volatile, but... once I cool down, and can see my therapist's helpful intentions, it gets much easier to continue with her and trust her.