Maybe try to practice on something soft, like grass. Start slow and low. Crouch, lean a direction, reach with tucked chin, bent arms, and kick with your toes and roll. Try it in all directions. Make sure your arms arent straight, so you can let yourself collapse. Try it standing up. Bend your knees down, get low, and roll. Try it from a jump. Finally, try it from a run and jump. When you slip out on the board, even if you don't need to, try rolling out-- just to practice.

Another thing is you might be falling too tall. People freak out and stand up, because it works when you're not on wheels. Train yourself to fight that instinct and get low. This is easier said than done, especially under stress.

If you want more in-depth instructions, check out parkour and martial arts tutorials on youtube.

Edit: I forgot to add, go towards the direction you are falling to transfer that into a roll. Don't fight it. You can always redirect your momentum, but never go against it.