Are you able to gape or is your partner able to? Is it something that occurs naturally or do you have to learn how to do it?
For me it’s just a matter of a lot of action and the anal sphincter relaxing! For me, I started training with dildos, gradually thicker and thicker.
Frequent back door play helps. One of my friends likes anal but neither her husband nor her boyfriend want to do it. So, when we get together, which isn't all that often now, it takes a little work to get her relaxed and wide enough to easily take things without too much pain. But once she's warmed up and lubed, it's a bunch of fun.
I know my ass gapes when I’ve been taking it long enough to relax and enjoy it. Once I’m gaped a guy can take it in and out much easier than that initial penetration.