How long will it take for me to learn how to do a backhand spring?

I have taken tumbling before with a very good spotter and coach. I had almost had my back handspring and when she was only spotting me on my leg, she moved and was no longer able to teach where I live. I then started doing backhand springs on my trampoline and had them very good, my best friend had spotted me in backhand springs a couple of times and then me and her didn't really focus of them very much and I ended up going sideways on the trampoline. My trampoline ripped and now I go to my friends house to practice them on the trampoline... just the other day my friend said she thought I was ready to be spotted on the ground, (and it had been about 7 or 8 months since i had done this on the ground and my friend is much smaller than me, but very is now working on her back tuck) so it took me about an hour to build up my confidence when we finally got her little sister to come and help support my back and make me feel more comfortable I was a little bit too reliable and I didn't jump, I basically just fell into their arms. My friend was a bit angry that it took so long for me to do it, so by that time she was ticked off, after she helped me up she walked off angrily and left me there waiting for another try.....what should I do if I really want to just get spotted on the ground and then work my way up to not being spotted on the ground? I just need some tips on what to do to feel like I'm ready to get spotted on the ground with my smaller but more experienced friend. someone please help....