in light of a breakup, can you help me understand what transitioning feels like?

I'm coming from a place of trying to understand just how BIG transitioning is.

I'm 25F, my (ex)partner is 25FTM. When we got together (Aug '21), he wasn't planning on taking hormones or transitioning in any way other than socially. i need to set the scene of our relationship: within 2 weeks we told each other we loved each other, I felt more for this man than anyone ever before. within 2 months I visited his family, and we revealed to each other that we felt we were "the one" and moved through our relationship with that mindset. we had a lot of struggles (mental health, health, career/school) between the both of us, but we were still so committed to each other. I encouraged him to look into transitioning, and he decided he wanted to go on hormones.

once he started HRT, everything shifted. the doctor warned him to not make any big life decisions in the first few months of T, because his mood would be all over the place. but after 6 weeks of being on T, he broke up with me completely out of the blue, telling me that he couldn't and didn't want to be with me, didn't feel like he was in love with me. he told me he still loves me and wants a future with me, but can't. he told me he has too many things he needs to figure out in his life right now, and that he can't give me false hope that we can get together in the future.

I have bpd, so this DESTROYED ME. I feel like I'm dying every day. But I love him so much and want to support him. I know he has an insane amount of stuff going on rn (transitioning, depression, chronic illness/disability, grad school, not very supportive family, etc), but I don't get how you can just Lose Feelings so suddenly? He's never been great with identifying feelings (and causes of them), and if he's stressed he completely loses his ability to access emotions. He has asked for a little more space, which is incredibly hard for me, but I want to support him. and the biggest way for me to do that is to try to understand.

So, I'm looking for understanding with what transitioning means. How does it feel? does it mess with your emotions? how long did it take for your emotions to stabilize? does it affect your understanding of who you are? what are physical, emotional, social, etc effects? is it something that really requires you to take time alone? when did you feel fully like yourself?

- I mean no disrespect or like, invalidation in any way when asking these questions. I know it could just be that he really did fall out of love with me, which happens in relationships. but it felt so sudden & so aligned with when he started HRT that it feels to me something is there. please tell me if I phrased anything wrong or said anything disrespectfully. Thank you all in advance, I'm really trying to learn and understand