How often is normal?

I'm 22 years old and am currently in a 2 year long relationship. In previous long term relationships i'v had sex on average 2-3 times a weeks but this one is considerably less. To begin with we would have sex maybe once a week but for the last one and a half to two years its been more like once a month, if that.
Is it ok for me to be upset about this? Not only is it physically frustrating for me but also its really affecting me mentally. I don't really know what to do. We have talked about it and she says its because she is scared. Not because of the sex itself but because we havent done it in ages. But then, surely thats not a reason because why would we not have had sex in ages in the first place? Also, when I try to get things started she just stiffens, i barely even get a response when i kiss her on the lips.

Please help

Seems there's more the matter than just that, I mean girls normally don't stiffen up when their guy kisses them after a two year long relationship. Did something happen, is she very selfconscious at the moment, maybe she doesn't feel like she's attractive anymore? I knew that, way back when I gained some weight it affected my sex drive negatively because I couldn't imagine that my guy still found me interesting. After losing all the weight I felt much better and the sex became more often. But for me, after only two years, once a month is way way too little. I mean, after about 6 years me and my ex still had sex about three times a week, and if we both didn't have busy schedules, more often. depends on the couple though, everyone's different.

Sex once a month would definately not be enough for me. Even after 2 years, sex should not slow down that much. Maybe she's cheated on you and feels guilty about it? Her excuse of "we haven't had sex in ages" isn't very logical excuse, you should try and dig deeper. Maybe she doesn't feel attractive to your or you're not giving her enough attention? Maybe she's really scared of getting pregnant? Try sitting down with her and discussing why not and try to ease her fears whatever they may be.

Females normally retreat from sex and when they feel their needs are not being met in other ways.

Are you sure you are treating her right?....LOL

If sex was all my guy talked/thought about, then it would put me off having sex too. I'd feel like a sex toy and not his gf or wife....