You need to train your hole to stretch and relax. I didn't bottom until I was forty. Here's what I did.
I got a set of s/m/l butt plugs, a quart of lube, a lube injector and an enema kit.
I cleaned out, lubed my hole and butt with the injector, lubed the smallest plug and slowly got on it. Gently.
Here's a good tip: to relax your sphincter muscles push like you're defecating. What allows poop out allows toys or cock in.
Stay on the butt plug only as long as comfortable. Might only be a few minutes at first. Gradually increase the amount of time you keep it inserted.
When you get used to the small plug to the point it's not longer painful graduate to the medium plug. Do the same until you can take the large. I took me two weeks of daily practice to get used to it and learn to relax my hole to the point where penetration was pleasurable not painful.
Ever wonder why butt plugs come in a set of s/m/l? This is why. Training .
Your butthole isn't used to being stretched open so much for so long. Once you get used to it it's easier to relax and take toys or cock.
I'm a top who seldom (but loves to) bottom. If I don't bottom for a long time my sphincter muscles tighten up and I have to train all over again but it was much easier to do once I learned how.
Good luck and enjoy it!