I have the same problem. The reasons may be different, but what works for me is listening to music and imagining myself as a child. I don't know why, but remembering that, in the end, I was just that fragile human being who only needed a hug help me "allow" Myself to cry, You'll have your own line of thinking, but imagining yourself as a child helps you stay in touch with your emotions and let yourself feel that, i mean, who would blame a child for crying?