We've already had sexual intercourse, and her masturbating makes me feel like I couldn't make her happy in any sexual way. I need help or understanding.
This is a common problem in relationships – although it's more often the woman who's bothered by her boyfriend's masturbating. I think you've already figured out the cause of your concern: it makes you feel less adequate. Sometimes we – and I'm talking about both sexes here – like to think our partner's sexual gratification is dependent on our own ability to provide them with pleasure. But it's important to understand we're able to provide ourselves with pleasure in different ways when we're by ourselves. Not necessarily better or worse, just different. And for many of us, we feel most gratified when we're able to enjoy sexual pleasure in a variety of forms.
Ask yourself this: Do you masturbate alone during periods (days or weeks) of sexual activity? Why? Your girlfriend feels the same way. In this respect – the desire to seek out different types of sexual gratification – the two sexes aren't all that different.
I'd say you should feel happy and lucky you have a girlfriend who's in touch enough with her body and sexuality to give herself pleasure. Provided it isn't interfering with your time together, I'd recommend you let her give herself pleasure whenever she likes.