That's really great news, I'm sure now she realises you'e not to be feared she'll be looking for you to make a fuss of her, your older mare will help as well as she will show she's not scared either.
I am sorry though that you feel in any way responsible, sometimes we have to go with what seems right at the time, if no-one parted with their horses just in case things didn't work out I wouldn't have mine. I was glad that though I had never had my own horse I was able to buy my arab, he was and is still very much loved by the lady who let me buy him but she did take a chance. I know from the tone of your posts that if you thought she was going to anything other than a caring loving home you wouldn't have sold her.
This next comment might sound harsh and nasty but I don't mean it that way but we are all animals and all of us have rough patches from time to time, that's what your mare has had but she is back being loved and cared for now and she is already responding and before long she'll be as happy and healthy as she was before and she will probably love you all the more for being the one to help her over her bad patch.
What is her name though please don't feel you have to say as this is an open forum? When I went to see Lady I can remember having to ask her name as the woman selling her never once used it which I thought was a bit sad and when she came to us and we called her she didn't respond at all, she does now though, I think 'Lady' is horse for 'there's more food if you want it'.
Can't wait to hear about the next baby step on the road to recovery.