i actually just taught myself backbends...it took about a month, but its worth it
I just walked my hands down the wall until i was in a bridge....every day...twice a day
and i held bridges for 10 seconds at a time
and tried to rock back on them
as i got closer and closer to doing one, i put an object (like a stepstool) that was about 6 inches from the ground and tried to c if i could do it then...
then, once i could, i put down books (4 inches from the ground, then 2)
and then one day i realized i could do it!
I dont know if it will take u that long, and i cheated and skipped some on some days, but for the most part i did it everyday
and for banging my head, I put pillows down sometimes but mostly did it on a really soft carpet without socks on so i wouldnt slide
also...try not to bend ur arms the second they touch the ground...try to lock them if u can- then u wont be able to hit ur head cuz ur hands will prevent ur from falling
sorry if that was really long and covoluted, but i hope i helped