Whay my coach has us do when we have a fear of a dismount on a beam. Is first try it normaly, if you don't go for it, turn around, walk to the other end to the beam, walk back and go for it, connected to your walking (don't stop). You could also do your dance from your routine into it. Sometimes what helps me is to imagine your self in a big competition. Can you compete it with a crash mat? Where I live we are allowed. Just remember it is the same skill, you are doing it the exact same way. The one thing not to do is just stand there thinking, you will just make your fear worse. If you are not ready to do it take a step back and work your way back up.
By crash mat I assume you are doing it onto a 8" mat. Are you trying to get it on a 4"mat or a 1" sting mat? Just imagine that the crash mat is not their. This might work better if the surface you are doing it onto is the same color of your crash mat. (It's a mental thing.) Take it onto a medium hight beam without a mat first.
Maybe you could try a different dismount. Maybe something backwards would be easier for you.