Sexual Advice

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Please help me /adv/

I've been dating my gf for over a year now and something big has changed recently. During the beginning of our relationship we had sex constantly, as with every new one. More days than not we would end up together. This lasted a few months, and then it tapered off a bit, down to once or twice a week.
Lately though, in the past couple months, things have gotten significantly less intimate. Less playful touches, less long kisses, and most noticeably: less sex. What used to be once or twice a week is down to once or twice a month. It's not due to wanting to end things, which was my immediate reaction, because all of this happened almost immediately following her wanting to get married. We are now engaged, yet I feel awful frequently. 90% of the time if I try to get intimate she says, "not tonight" or starts a fight. They aren't bad, and I'm not an asshole. If she doesn't want it then I always try not to push. But it's starting to take a toll.
I honestly don't know what to do. If the answer is to just wait then I'm fine with that, but if there's a damn thing I could do to make the situation better, I'd love to hear it. Again, I'm not above changing myself so let me know if I just need to get over this.