I'm kind of having the same issues with my partner - except it got to the point where me "just going along with it" wasn't good enough anymore. And then I couldn't do it at all. Now we're at one of those cliffs...aiyee.

I personally find sex pretty tedious and mechanical, even if I love my partner. It's just not a part of true intimacy for me. I'd rather be doing a lot of other things, and yep - I do sort of mentally drift off.