It is really difficult when a parent tries to stand in the way of their childs relationships, and of course their happiness. If the b/f or g/f makes their child happy then you would think that would be enough. Even though you are sad because your g/f can not come over I would try to have your spirits up when you speak with her. She may get off the phone and tell her mom how sad you are and how sad she is! Probably out of frustration but her mother sees her as sad once again. Will her mother allow you to visit their home?
Have you ever gone over and asked the moms permission to spend some time with her daughter. If not you may try.
Sometimes it is some work to get what you want. Most of the time it is worth it. Good luck to you and your girlfriend.