The most important lesson

That I could wake up from a reactive life, with emotions that clouded my judgment and destroyed my relationships with others. In waking up, I could live consciously and intentionally, making choices rather than having reactions.

I've learnt that it is sometimes possible for me to verbalise my thoughts and feelings. Provided, obviously, that the person who listens is paid to do so.
I've also learnt that in my T's office it is ok for me to experience emotion, even when I can't put words on it. I haven't really put this last one to the test yet.

I've learned that emotional intelligence is one of the primary qualities I want in both friends and significant others. I want to be surrounded by people who are in touch with their emotions, emotionally available, willing to talk about how they feel, what they need, etc, and have the capacity to listen to me, empathize with me, support me, and partner with me. I can offer that to others, and I deserve to receive that in return.

That's I'm not to blame for my mother's disappointment in life and depression and it's not my purpose in life to keep her company in this never ending mirroring...

Well, hiding is not the best way to describe what a therapist does, but they do keep their own needs out of the therapy relationship. It doesn't make them ingenuous, just thoughtful and professional.
Constantly hiding and denying your needs in life though, that's a whole different animal...