Is it wrong to make my girlfriend do things she doesn't want to in bed?

It is "wrong" to force someone to do something they don't want to do. Sounds like you're in a challenging situation where your gf is reluctant but sometimes willing to go farther. But it sounds like you have a decent working arrangement where she will tell you how far she is willing. If it were really painful for her I would imagine she would stop you...as she seems to do on occasion. So keep in touch with her feelings. Honor them.