It felt really painful to read what you said about him mentioned once per session that he won't be here for you forever.

I would most likely do the same thing you do-- ignore it. Ugh. I have tried to talk with mine before about my fear of abandonment, the notion of not having him anymore, the fear that at any session he could say, "Well, I'm moving across the country..." But it became too real for me. We started to talk about it, but it was like he really did tell me he was moving or something... my reaction was getting that bad... had to switch the subject; never touched upon it again. So I can see what you would ignore it.
The thing about the therapist leaving as part of the healing process... I would assume that statement refers to a point in which the client has healed sufficiently? It's no longer part of the healing process if the leaving occurs when the client is still in need of the relationship. It has happened to me before. The therapist leaving before we even scratched the surface... felt horrible.
Hang in there, Alex. Do you feel like you are healing?