Now anal is not my thing, so maybe this advice comes easy....but I'd drop it. Yeah, it sounds like the old "bait and switch", which would pizz me off, but how important is this one aspect if everything else is fine?
It can be painful for some women, and likely even for most women if it's not done right. Personally if she's not getting a great deal of pleasure from it (either from satisfaction from doing for you, or her own physical pleasure), then I'd not want to do it. I liken it to blowjobs....there's not much physical pleasure in it for her, but she seems to genuinely enjoy doing it, so "I'm good". If she were to treat it like a chore and complain that she doesn't like it, well, I'm not going to be pursuing it from her. If she stops enjoying giving that to me though, that may point to bigger relationship issues.
Now if she starts the bait and switch with all things sexual, then I'd have a serious problem with it.