For over 20 years my wife resisted anal sex. Once she agreed to be patient and allow me to prepare her for sex with adequate lubrication and dilation.
Initially there is still an adaptation to having things go 'in the out door' - but after about 1 month, she's like a lifelong pro.
The primary issue is mental. Accept that this is something that you are doing for your lover - but that will also contribute to your pleasure with patience and care.
The first step for us is to make sure she has an orgasm using some toys - often with some anal penetration using a finger or small vibe.
Then if we are going to have anal sex, I will apply a lot more lube, reinsert a finger, then a second, then a modest size vibrating butt plug - something with a maximum girth of 1.25-1.5" diameter.
Once I am able to insert it, I then partially remove it - providing gradual anal stretching. Usually after about 2-4 minutes she will tell me she is ready.
I add a new coating of silicone lube to both of us. Typically I place her hips on our Wedge pillow - then enter. Once I enter, I do not move - but allow her time to adapt. After a minute or so - I can begin slow thrusting.
Side comment - it helped that she did anal play on me - so I knew firsthand what she was experiencing - both the initial discomfort - and the pleasure that ensues.
Final comment - by having her apply her Mystic Wand to her clit, she was able to orgasm during anal sex.
Good luck - patience and there is no such thing as too much lube@!