Can you do a back tuck on trampoline? If not, can you do a back drop pullover on trampoline? If not, can you do a back drop? Can you do a backward roll of the edge of a mat into a pit?
In general terms here's what's happening with a mental block:
Your brain has something called the limbic system which deals with basic survival drives. The limbic system's main purpose is to take control when your immediate survival is threatened. A mental block occurs when -- for whatever reason -- your limbic system decides that a particular skill or category of skills is a survival threat. The rest of your brain might reasonably and logically know that you'll probably be fine, but the limbic system doesn't deal in reason or logic.
So how do you deal with this? Patience. Break the skill down into components, try to find the component that scares you, and work that component in the safest manner you can.
Is it the going backwards part that scares you, or is it the inversion? Do some backward rolls and see how you feel. Then backward rolls off the edge of a block into the pit. Then pullovers to front drop on trampoline. Then Back drops into the pit. Then back drops onto a wedge mat and immediate backward roll. etc, etc, etc.
The idea is to break the skill down into parts as small as possible until you find the smallest possible part that you're blocked on, and then work on that in an absurdly easy and non-threatening way, and work your way back up from there.
Try your best to avoid situations which make you feel stressed or afraid about the skill; stress and fear are what feed the limbic system.