As others have pointed out, you have to relax, be patient and use lots of lube.
What always seemed to work for me when trying anal for the first time with a provider or ex-GF, was to let her do the pushing at her pace.
What happens more times than not, is even if the lady thinks she is completely relaxed, the second the guy starts to enter her ... she tenses up. I used to just lay back and let her get on top, or we would get in the K9 position and i would remain still while she pushed back against me. She controlled the pace and she controlled the depth of penetration. If it was to much, she backed off.
Once she took her time and relaxed at her pace, I was able to get inside her. After full penetration and she is comfortable, then let him take over ... SLOWLY!!
When she's pulling the covers off the bed and howling at the moon ... enlightenment has been achieved.
Good luck.