I used to have the EXACT same problem with my backwalkover, about 2 months ago! Now i am am doing BWO BWO's and BHS's on high beam by myself! This is what i did...I got panel mats on each side of the beam, and would slowly unfold panel by panel. Then, when i got to just 1 little panel, i would slowly slide them centimeter by centimeter away from the beam (i know...i was REALLY scared!). Once i would do it on the low beam, i stacked mats under the medium beam, AND put panel mats on the mats so it was just like how I started. I slowly unfolded panel mats and took away mats until i was doing it on the high beam!
And i didn't have a spot on any! (my coaches don't spot on beam anymore because we get to attached to the spot

) and my coaches method works! I have gotten SO many new skills on beam this way, and you will feel a lot more independent and proud of yourself by doing things (only on beam) without a spot because you can just work your way up!