It truly requires commitment from you and your partner to make it as painless as possible.
I started out as a total bottom, and the first time, it was with a guy who knew what he was doing. He got plenty of lube in me and on his cock and watched my reactions carefully. He eased it in gradually, and left it in when my sphincter was being stretched. It hurt at first, but I became accustomed to it rather quickly, as we stayed stationary until the initial wave of pain had passed. There was very little pain after that, as he gradually made his thrusts deeper so that I could acclimate to the feeling, and it felt so good. He made sure to be expressive because of my love for making other people happy, and I was so focused on how good the two of us were feeling that I came pretty quickly, setting him off.
My next encounter was the next night, with another guy, who didn't care about my pleasure. While his dick was shorter, it was fatter, and he didn't even lube up or give me any sort of warning. He just shoved it in and went to town. It was painful for far longer than it needed to be, and it only really started to feel good when he started to ooze copious amounts of precum in my ass. He kept pounding me for maybe twenty more seconds and then flipped me over and his cum gushed all over me. I realized that I have a huge fetish for cum and stayed the night. We did it again in the morning before I left for work, but I couldn't even enjoy it because my ass was so sore.
Eventually my body learned how to adapt to bottoming. I wish I knew what words to tell you, other than it takes time, and effort. Also, making the motion for pushing out while he's pushing in actually helps with getting it in deeper, since it widens the pathway. But my most important point is that it takes two to make it a pleasurable experience. Do it with someone who wants you to feel the least amount of pain and the most amount of pleasure. If you're choosing to do it with someone you love, then your goals should not be to get off, but to make the experience enjoyable for the other partner.
Btw, it turns out I'm versatile. And I'm really up for doing whatever I know will please my partner. Lately I've been topping so much that I'm starting to miss the awesome feeling of being a bottom.