Stables can be bitchy and usually eventually anyone can become close friends with people at the stables after a period of time. The other girls are probably just uncomfortable by you getting in their business, they probably like the independence they have at the stables and feel that you are impeding on that.
I suggest that you only become friends with the other parents and leave your child to become friends with the other riders by herself, limit yourself to saying only "hi or bye" in passing to your childs friends. If you say she doesn't seem fazed by you changing and just dropping her off, I suggest you let her do her thing as long as she is happy. She will really appreciate the experience this gives her. Only offer help if she asks for it, but tell her beforehand that she can ask you for help.
If you buy a pony realize that won't instantly make her more popular with the other girls. Riding is a great sport for young people as it teaches them more respect, responsibility and encourages an eye for safety than any other sport, if you trust the owner and the other girls then let your daughter get as much out of the experience as you can.
If you want to start riding again - do it! Don't live through your children.
If other girls want to join her, then they will, and if not, she'll still be okay.