Feeling unattractive yet vain

Here's the thing:

My girlfriend and I live together. We have sex occasionally, but I feel it's not enough. Now, I don't wish to have it ALL the time, No. Just a few times throughout the week. I find that's a decent amount. After all, we are young still, (late twenties), and sex between two people that love each other as much as we do, is a wonderful act among other things to experience. My girlfriend always has an excuse for not wanting to have sex, though. Either a canker sore, or an inflamed rib, or allergies, or sleep, or anything. As a result, we don't have sex, and right about the time she feels better, her period arrives, blocking off an entire week. If I'm lucky, we get to have sex once, if not twice a month, and I feel I get it because she "gives in" and patronizes me. I feel as if it's not because she enjoys it the way I do, or sees it as a wonderful experience, I feel she does it mostly to give me the feeling of completion she feels I need... If that makes sense. Like,

"Fine, here's the sex you want. Happy now?"

and it leaves me thinking,

"Does she even want to?"

Tonight, I showered, then we watched the NBA finals, and then she showered. We got to bed relatively early, and after trying to make a move, she hesitated as if she were tired, so I backed off... annoyed. She then felt bad, as she sensed I'd gotten into a bad mood, and apologized.

Part of me says, I should just chill out, and think about her feelings, but at the same time, I have hormones, and a self esteem that questions whether or not she finds me attractive. Sometimes, I think about cheating, to confirm whether or not, I've just become unattractive to the opposite sex, but I don't have it in me to do so.

I really try not to pressure her, but I feel I'm the car dealer trying every trick in the book, to get something that only I can get, and it makes me feel lousy. I feel low and cheap. Worst of all, I feel unattractive. Oi.

What should I do? :-/