If her drive is higher than yours I don't see why you should be concerned if she is masturbating. If she is denying you sex and prefferring masturbation, then you would have a cause for worry - but if she just wants it more than you do - what harm really in her pleasing herself?
Orgasm isn't easy for many women - some NEVER achieve one , even through masturbation. The fact that she is sexual enough to want to have one, trusts you enough to masturbate in front of you - it really should make you feel good about yourself not bad.
Its not your inability to "make" her have an orgasm, she is helping you, womens bodies are complex and need a lot more specific stimulation to achieve orgasm then most men do. Watch her, if you want to be the one to do it for her, try to learn how she touches herself when you are penetrating her.. and use your fingers instead.
Expecting her to come with your penis alone and feeling bad if she doesn't is asking a lot as for a lot of women this is not possible. You shouldn't feel bad, its not at all you, its her wanting to experience the same pleasure you do when you orgasm, why would you want to deny her that just to prove a macho point that you are capable of doing it by yourself?