Your problems resonate well with me, I have a similar life situation.
We've been discussing about polyamory for a long time, my partner is also trans* and I think that not being cis-gendered makes him think that he can't find other partners, it's too risky for him etc. He has a low self esteem. I feel like much of that is his own internalized transphobia (omg he's so hot
If you feel like discussing this privately with me, feel free to private msg me!