I don't think the issue is hypersensitivity per se. I'm guessing that you're partner is extremely nervous and up tight. (Excuse the pun!) To counter this I would suggest that you need to spend much more time reassuring her and boosting her confidence, which can be achieved initially by discussion and talking to her as you commence and continue with play. Most importantly though, you need to slow things way, way, down, and spend numerous sessions just caressing her anus externally to get her body and mind comfortable with being touched, but also to allow her to get in the zone. In time, she will gain the courage to allow you to penetrate her, but don't make the mistake of thinking this is the signal for total penetration. Instead, and again over many sessions, just slowly and gently progress from external touching to caressing just inside the mouth of the anus, to very shallowly entering the first sphincter. Your radar should be on high alert as you do this by listening to her breathing and her general body language. In time, when she is feeling ultimately comfortable with being slightly penetrated, and ooohing and ahhhing contentedly, you can slowly begin venturing deeper. Hopefully by this time she will be somewhat addicted to the sensations and enjoyment her butt can provide, and penetration of her second sphincter will proceed as a matter of course and without any further concern. One word of warning....when you're discussing your anal adventure with her, do not emphasise or build up the issue of penetrating her second sphincter as a massive mountain to be scaled. Just let it happen naturally without any hulabaloo.
Good luck, and keep us informed of your progress.
Lick her hole, fondle it (on the surface), talk dirty. Tell her how sexy her ass is, and how you want to feel your dick in there. How you want to enjoy and fill up all her holes. How you want to give her a new type of pleasure, take her to another zone. How you want to make love to all of her, including her ass.
In my experience, fingers and sex toys are nowhere near as comfortable as a real dick, even a big one. Dicks are soft but hard at the same time, soft enough to not hurt, hard enough to feel good, and warm. Sex toys felt very uncomfortable in my ass. Fingers scare me, I hate it in either my pussy or ass, and it has a chance of scraping the insides even with trimmed fingernails. If you need to use a finger, use your thumb, and feel around her rectum for her g spot. Either way, nothing beats real dick. It was easier "training" my ass with an actual dick, and much sexier and intimate that way too, than using fingers or toys. Maybe this is the right technique for your girl.