Hmmm
Interesting predicament coaltrain! lol
Anal sex, is all about comfortability (for her!) she has to be completely relaxed, trust you implicitly & know that you'll be gentle.
This will take some effort & concentration on your part. No pounding, no sudden thrusts & no 'letting go' with the force of you're arousal, you need to let her lead completely.
Am sure you've done all of those things of course, but mentally, even though she wants it, if she's thinking "this hurts, its going to hurt" - that sphincter muscle will tense up (a natural & normal response) and that penis, big or not, will never get in there! lol
My advice?
Stay away from 'doggie' style with anal sex, especially the first few times.
Try it in missionary position, her on her back, bring her to arousal (with fingers, tongue, toys, whatever you got!) lol It's a good idea to bring her to at least one orgasm before you even attempt anal sex (post orgasm will see all sexual muscles relax, there's also a rush of endorphins after orgasm, which actually can make a hint of pain, quite pleasurable

)
Then get her to masturbate herself (fingers or toy) & either you, her, or both of you, assist in sliding you into her, verrry slowly. This process, will test your perseverence & patience, but she needs to keep masturbating, the entire time, to ensure her mind is on her arousal, rather than thinking about how painful it might be.
Once the head of your penis 'pops' inside her bottom (& that will be the most painful part) ease yourself in very very slowly. Follow her lead, let her control it, the depth, the movement etc. and encourage her to keep masturbating, whilst you're inside of her. Its visually exciting for you & will be deeply satisfying for her
You may need to work up to it & it may take a few good, determined tries. But I can say, hand on heart, she will have, easily, the most intense orgasm of her life
Good luck! lol