Dude.
You HAVE to listen to what people here are telling you. We have been where you are. You had fantastic sex at the start of your relationship. You thought, this is great, you're compatible, she's wife material. Now, it's dwindling already and you're not even married yet. What will it be like when you are even further down the line and you add children and ageing into the mix?
This will not improve on its own. In fact, the longer it continues, the more it becomes "just the way things are" in your relationship and the harder it is to change.
I agree with @LochSunart , if I'd been told this at your stage I doubt I'd have listened either. The sex used to be so great, and she seemed to enjoy it so much! Getting married will reignite the spark surely. No it won't.
Talking won't help. There isn't really any talking to be done, because she doesn't see the situation as a problem.
If it was the case that, like you, she was longing for the sex you used to have, but if only you could stop doing whatever it is you're doing wrong that's putting her off sex with you (but somehow isn't quite bad enough to warrant calling off the wedding), then she would tell you.
I think that for many women, having lots of great sex for its own sake is a thing that is strictly restricted to the start of a new relationship. The mistake that many men make is to assume that this will simply continue for years down the line. But after that initial rush of attraction has faded, she actually has no real interest in sex, and wants it to be on a much more occasional basis (if at all) as you are discovering.
And this is okay! She is allowed to do this! BUT, surely she needs to marry a man who feels the same and who is on board with it.
Understand and accept that it does not matter at this point what sex USED to be like between you. Those days are gone as far as she is concerned. She is showing you NOW what it will be like once you are married. Listen to her.
You do not owe it to her to commit to a lifetime of this.
I don't know when you're meant to be getting married. Yes, calling it all off will be embarrassing and expensive. BUT. That will pass in a short while, and it will be much less pain and heartache than divorcing many years down the line once children are involved.
The later you leave it, the worse it will be.
You need to man up and do it.