Twisting in back handsprings

[htsgymnast]

Hi, I've been trying to learn my back handspring for about 3 months now, but I can't seem to get it right. I've been practicing with those round/octagonal barrel things and I can do it fairly well with them or when someone is spotting me. But when I try to do it on trampoline or tumble trak without spotting I consistently twist to the side or rotate around too far. Either way, I end up landing on my side or stomach. It doesn't hurt, but it is very discouraging since I have been working on it for so long. Are there any tips or tricks to help with this? Or are there skills I should learn first? (I already have my back walkover, but not my back limber)