That this man fills a need in you, I have no doubt. But I think it's a need to feel needed, and while you may recognize that, it's the very same need that's, basically, sucking the life out of you.
I applaud your efforts and I most certainly understand your conflict. But, speaking from a purely objective standpoint, personal fulfillment should ADD to your life, and this is SUBTRACTING from it.

As far as his daughter goes, she's probably in denial of both how difficult it is for you, and what the day-to-day care is like for her father.....Let's face it, she goes home after soothing her conscious with a visit to dad. I am not speaking badly of her, but that's what these visits are for family.....a way to assuage their guilt and reinforce their denial.

Please don't think harshly of me and I'm sure that there are some families who are truly dedicated, but I've seen too many "Christmas" visits that last an hour and listened to too many explanations along the lines of "I simply can't deal with it, you understand"....

And I TOTALLY get the resentment. It's a thankless job, whether you love him or not. It doesn't make you a "bad" person, it makes you human.