Suzuki,
Having gone through a similar experience, I can relate.
Re: your partner's expressions of terror/comfort; excitement/despair - Yes, my trans-identifying husband went through that too. At a certain point we mutually decided to call it quits. The very next day he detracted everything saying, "it's not fun anymore." I think the forbidden aspect has a certain appeal. Also I do not understand why so many people are declaring that they are the wrong gender, just over the last decade or so. I think it's because the general stigma has decreased, yet this is still considered edgy and different. My husband historically craved attention from strangers.
Re: your frustration over not finding a way to support him, and his reaching out to others for comfort and celebration. If your situation is anything like mine, you will not win. (And maybe that's a positive thing.) If his transformation is against your beliefs and what you want in a relationship, you have no duty nor responsibility to support him. And if he is reaching out to the trans community for comfort and celebration, THEY are not going to support YOU. I found this out the hard way. My husband's trans therapist told him if I didn't like what he was doing, I could go f**k myself. Real nice. Not to sound bitter (well I guess I am), but I think they want more people to be like them and they will encourage anyone who expresses an interest in this lifestyle.
By the way, my husband revealed all this to me 10+ years into our relationship, when we were both in our 60s. We are divorced now.
You may have additional obstacles to deal with as COVID restrictions are loosened. My ex was obsessed with drag queen shows and gay pride events - which I am sure will be returning soon.
Good luck. I am sorry you are going through this.