It's not just a fantasy, is it?
Depends on the therapist, but mine, for example is quite willing to fill the good-enough-mother role- to be consistent, caring, supportive, committed, etc.

Just because she can't always be there, or be there forever or be perfect doesn't mean she can't help me a lot and doesn't want to....

I wish I had more advice from the perspective you're looking for, but my reality is that the attachment is difficult, but beneficial and like you say, the premature withdrawal isn't the best way to work it through in my experience.

It might help to know what you mean by "too dependent" What harm has come to you from depending on your therapist? Maybe from there will come some answers or a way for others to give more concrete help?

I'd probably be asking myself, what could I do to feel comforted between sessions, given the time limits, like whether she could provide me a voicemail to listen to, or a comforting letter or a song to share, etc. That it's not bad to feel so needy if you're not actually doing destructive things because of it, but rather, a signal to work on getting what you need...