I feel that. I also have this problem. If it's about my own emotions, my pain, my sacrifices, doesn't matter how much pain I feel, how bad my thoughts are I rarely cry. It's bad and sometimes I wish I could cry but I can't and then when everything boils over for months, when I'm at my worst or something triggers me, I Maybe cry but then I cry so hard that my crying makes me feel even worse. It's f*cked up.
What I have noticed that it's easier to cry if I feel empathy for other's stories. For example seriously sad movies or games about loss and death can trigger my empathy, my own pain about said topics to a point where I cry "for them" or the sad situation I'm witnessing. It doesn't work always but there's a few movies that are prone to trigger me actually.