You are doing what you can. She is 15, most 15 year olds aren't yet fully comfortable in their skin (without being trans as a factor). Kids at school might be dicks, she might have something else going on, she may not be comfortable talking about it. Just tell her you love her, and that you are always willing to listen. Hormones are going to make her go through puberty, teen girls are challenging then.

My 7 year old daughter will never walk without equipment. She'll always have a wheelchair. Years of physical therapy and surgery haven't changed that, and at some point she is going to lash out over being different. It won't be forever, it isn't anything I have done, kids just have to process stuff in their own time. The teen years are hard enough being a "normal" kid, all we can do as parents is try to make sure that they make it to adulthood as well adjusted as possible. They'll still have bad days, they'll still mess up... but we can at least let them know they are supported.

Hang in there, and just keep on trying your best. That's what separates the bad parents from the good ones, trying even though we'll make mistakes.