I was in almost your exact same situation. LTR that moved in and then the spark left. People are going to tell you that your girlfriend "just doesn't want sex as much as you", or some other shit about different libidos. Im here to tell you that is complete bullshit. This might be hard to hear but this is what is going on. Your girlfriend is no longer attracted to you. Do you think if ryan gosling whisked her away for a week should would turn him down because "sex just wasnt important to her"? Fuck no, she would be slobbering all over his dick every chance she got. She still loves you, because you validate and support her, you just don't give her that primitive sexual lust anymore. This is important YOU CAN'T NEGOTIATE DESIRE. You need to immediately stop whining about not getting enough sex. Nothing dries up a vagina like a man who whines and complains about sex. These are the steps you need to take:

Start working out. Getting a hot body is a quick and easy step to building attraction. Stop being so available to your gf all the time. Become more distant and she will miss you, then she will be more likely to get turned on when you are around. Start dressing nicer and keep your hair neat. Learn how to flirt and hit on women, and do it in front o her in public (within reason). People will say it's manipulation,but it's pretty well known that women are attracted to the man other women want. If she sees other girls being receptive to your light flirtations, it will kick in her primal instinct to lock you down with some hot sex.

Anyways, this was just a basic guide. But the bottom line is you've become to soft and lost your "hawt" manliness that you once had. Hit me up if you want more info. You have a lot of work to do.