Hi,
I don't really think a working girl is going to solve your problems. I would rather have a look around the internet and try and get help from a professional sex therapist, like Dr Eve for example http://
www.dreve.co.za/ Your girlfriend, hot as she is, must realise that you have needs as well and it sounds like she's being a bit selfish. Sure you're happy to do her the way that she wants but she must also let you do her the way you want. Who knows she might even discover other postions she likes. Your solution needs to start outside the bedroom by talking to her about how you feel, telling her exactly what you told us right now. Ask her what it is about that positions that she likes so much, and what it is about the other positions that she dislikes. She doesn't like masturbation, maybe you can suggest (you need to be tactful though) getting a vibrator that she can use privately at first and then maybe with you. Things may not change overnight but letting her know how you feel would be the first step.
Good luck.