Where'd My Sex Drive Go?

Ally
I love my boyfriend. He's caring, considerate, funny, just an all around wonderful guy. When we first started dating, we would have fantastic sex. He loves going down on me and has an amazing dick, so why I'm having this issue is really stumping me.
I would say for at least 3-4 months now, my sex drive has been incredibly low to non-existent. I can only count a handful of times I've enjoyed sex with him, while the other times I'm just going through the motions, trying to trick myself into getting more in the mood, because I know how much he loves me and my body, and I want to make him happy.
I don't think it's anything he's doing. Like I mentioned before, he's very attentive to my needs and always makes sure I'm satisfied.
I always feel like something's off with me, like I'm too tired or not wet enough, or this last time I got so nauseous while we were doing it it took everything in me to not stop and run to the bathroom. 
I really wanna enjoy sex with him, because I love him and this could be a long-time thing, but I feel like every time we do it, I'm in my head worrying. I barely feel horny any more when I'm around him and it really upsets me. I have no reason not to be sexually attracted to him, he's the best!
TL;DR, has this ever happened to anyone else? If so, was there anything that helped boost your sex drive? 
Thanks in advance for any advice!

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