How to do a backbend??? Please help!?

i don't go to gymnastics but i am quite flexible (i can do splits on both legs) but i can't do a backbend properly! i can do a bridge and hold it for over a minute without a problem but whenever i try to go backwards with my arms up straight above my head, i fall backwards. i can do it holding my thighs and then throwing my arms up but i really need to learn it properly to lear a back walkover... PLEASE HEP! :(