Do a back walk-over on the floor and compare it to the ones you've done on beam....are they the same? If you were to do your "beam" back walk-over on the floor would it be as "easy" as the floor back walk-over, on the floor?......Be honest, aren't you doing a "version" of your back walk-over when you try it on beam, just to be safe.....Answer this question, If the easiest way to do a back walk-over is what you do on floor, how can the "harder version" that you do only on beam be safer????
In other words, do it the easy way, not the hard way. If it works on the floor line 95 out of 100 times with the 5 "bad ones" just a bit off, it will work fine on the beam if you just let it happen. Spotting's fine if you need it to keep your beam technique the same as your floor technique, but you'd be better off staying on the floor until you're "bored crazy and you just can't take it any more I'll do anything to make myself move on, nuts in the head" Until then use your time working lots of repititions while concentrating on going straight over the top and moving through the skill with enough speed that it "flows" continuously from beginning to end. Balance beam isn't hard unless you make it that way!