Hi! Ughh, I have been in your EXACT position a little while ago. Everyone on my team was doing back walkovers on beam and I was afraid to do mine without spot and was scared even with spot on high and medium beam.

I was really frustrated, but after I switched gyms ( other reasons) my new coach gave me some tips to help me, and they worked! I'll give these to you now.
1) You might have already heard this from your coach, but try mental choreography. Basically, think of certain "words" that go with the skill as you do it. I know this sounds a little confusing, but it would work like this : (before skill) "you can do this" during ( "arms up" "raise leg" "bend back" "look for hands" "over" "finish." Basically, as you do the skill, you would say these words. If you practice with these, the skill becomes automatic in your mind.
2)I know you're a level 5 so you probably don't have the level 6 routine, but doing a movement right before the back walkover can sort of take your mind away from the skill and the fear. If you ha a routine, this would be plie plie, but you might have to thing of something.
3) Increasing your back flexibility can be really helpful, too. At the time,I had an injured back, so it wasn't very flexible. as my back got better and more flexible, the skill became much easier and less scary. So , stretching bridges and arch and backbends might help you.
4) lastly, when you practice on floor (or on your beam), don't just go/fall back, think of it as if you were on beam, and have your moment of concentration, mental choreography if you decide to try it, ect. and, if you trust your coach, just ask for a REALLY heavy spot and just go for it. I know you've heard this many times, but with bwo's it's really true! It isn't so scary. Good Luck, Hope I helped you!