Hi, welcome to the forum.
A backstop mount is a good half way stage to learning to do a full freemount. That means bracing the back of the wheel against the kerb, and later bracing it against something lower, and then something even lower, or mounting on a slight downhill, until one day you can do it without bracing it against anything.
However, the technique I use is to brace the pedal against my own forward momentum. I do a static mount, but with a little forward push.
This is what I do:
Put the cranks at somewhere around 45 degrees, with the lower of the two nearest to me. I put the left crank low and the right crank high.
Standing with all my weight on my right foot on the ground, I place my left foot on the pedal.
I put my crotch on the seat. It’s not quite in the proper riding position because the unicycle is leaning towards me and the seat is pointing up at 45 degrees.
I hold the front of the seat (handle) with my right hand.
Now as I’m about to hop up, I push forward with my right foot. The effect of this is that the unicycle moves forward about an inch and the left pedal rises very slightly. I can now put a small amount of downward pressure on this pedal and it stops dead and makes a temporary “step”. It gives me just enough support to be able to step up onto the seat.
This works or me on any size of unicycle from 20" right up to 36".
There are other things you can do to help:
Practise riding slowly.
Ride at normal speed then slow right down to a stop, without dismounting, then start riding again.
Practise riding, slowing to a stop and doing one little hop before riding away.
All of these things will help you to develop the skills that you need to ride away from a freemount.
I’m guessing that you’re learning on a 20 because most people do.
The hub of a 20 is only 10 inches above the ground. The cranks are likely to be around 125mm which is 5 inches. Therefore, when the pedal is at its very lowest point, it is only 10 minus 5 = 5 inches off the ground.
When you’re using the technique I have described, the pedal will be a little bit higher: maybe 6 or 7 inches off the ground. Think of something that’s about 6 or 7 inches long: would it be difficult to jump that far?
One day soon, you will wonder why it was ever a problem.
Good luck.