I have a similar problem. It worried me that people would think I wasn't grieving when my parents died. Sometimes I'll start to cry and then it shuts down. I am more able to cry when I'm alone. I tend to not have"normal" reactions to emotionally charged events.

Some of it may have to do with family history. I lost an older brother when we were still in single digit ages. My mom never could cry after that. Not until about 40 years later.

Have you tried "practicing" with really sad movies? Especially the ones that end tragically? Also, have you discussed this with your bf? Maybe he can help you learn to be more emotionally vulnerable with others.