I'm new on this forum so I hope I am not too late for this post...
In my experience, having difficulty crying often comes down to a belief that you are not going to be heard or appreciated, or a judgement about crying, being 'weak' or 'girly' or something (funny enough, a lot of women have the subconscious belief that it is bad to be a woman and so it is bad to cry). That might not be what is going on in your case Liz, but I have found that that is the most common cases.
So affirm for yourself that it is ok to cry, and you will be heard and appreciated, that might help.
As far as getting the thing going, here are two ways. One is lie down and close your eyes. put your hand on your heart and imagine a little person in there. Open your mouth and let out a sound like 'aaaahhh', but mentally connect it to the little person in your heart, so it seems like that person, or your heart itself, is making the 'aaaahhh' sound. Do this straight for about half an hour, concentrating on your heart and how your heart feels finally being able to express itself. This is an extraordinary exercise even if you don't end up crying, but you probably will end up crying after a while. When you do, feel how good it is to finally cry and let it out all the way.
If that doesn't work, this is a bit more heavy duty. Lie down, again with your eyes closed. Have a rolled up towel handy. Put your hand on the top of your chest, just where your collarbones meet. Breathe in and out with no pause between the inhale and exhale, making sure that your hand on the top of your chest is rising up and down. Really nail it hard, like you have just sprinted a marathon. You will start to first laugh then fill up with anger and sadness so much it is almost painful. When it gets to that stage, put the rolled up towel in your mouth and scream (whatever nasty phrase you can think of. Scream until it all blasts out, then go for another round. Doing this will also get rid of a lot of stuff, but you will most likely end up crying as that is easier.
I hope that helps.