I usually can cry easily. However, there were a few years in high school where I would cry every night. After a while, though I wanted to cry and felt exactly the same as I did in the situations where I cried, there were no tears coming anymore. Pretty similar to how you described, it kind of hurt in parts of my head, though I don't remember how exactly. I'm not sure why that was, after not crying so much for a few years, I now do not have the issue again.

I'll say that I think you can start mourning even if crying doesn't really work. Crying is calming, but it's not necessary to process your feelings. Have you ever brought this up in therapy, especially the part about mourning? I think it could help to talk about it, your T might be able to show you different ways of dealing with everything or maybe they also have insight on why some people can't really cry. From my short googling, I've read that it can happen when you're too overwhelmed. Maybe that's a possibility for you? In that case, your T could help you regulate those emotions and with time maybe there's even tears.