You could try starting out a little smaller.
Do a standing forward roll but keep you back leg straight and end in the breakfall position on the floor. Be careful about trying to roll back up. As you get more comfortable with that you can give it a little more oomph
I've had pretty good luck with using a big yokomen-ish strike that I let carry through which carries the body up and over.
Good luck, practice safe.
Bronson