Ignoring everything else,
OP - use the right lube!
There is special lube made for bumsex which is thicker and lasts longer than the usual thin kind you can buy in the supermarket/chemist. (Reason being, the vagina produces much more lubricant than your bum, so the lube you use needs to go the distance!)
You'll need to order it online or you might be able to get it in Ann Summers or the like, not sure.
A PPs post on page one about working your way up to it with butt plugs is very good. I also use an anal douche beforehand so I don't have any anxieties about being empty, as it were.
If your partner is well-endowed, missionary with a pillow under your bum will prevent him going too hard or fast for you. If he's average or smaller, doggy style will work.
I enjoy anal and can sometimes actually find it LESS uncomfortable and painful than vaginal.
But horses for courses, of course we should all strive to meet our partner's sexual needs, but there's a difference between "this doesn't do anything for me but I know my partner likes it so I'm happy to indulge them occasionally" and "I really dislike this but I'm just going to have to put up with it." If you find it painful and uncomfortable even when fully warmed up, then you take it off the menu.