I'd been a top for years before I bottomed the first time.
We were both rather drunk, and he was really really big. We didn't do nearly enough foreplay, and I really wasn't as ready as I thought I was. It hurt quite a bit, and was not the good time that I'd hoped for. That said, I know that first time could have been much better, but drinking really got in the way.
I've since had much better experiences bottoming.