I am a 33 year old gay man who has had a difficult time bottoming since day one, never knew how to express exactly what the problem was, and have just figured it out. I'm hoping I can save some people out there the time it took me to make this discovery.
I've always been turned on by the idea of bottoming, but the actual act has been a mix of pleasure with a heavy dose of pain, and not a good kind of pain. I experienced a burning sensation that I always attributed to my ass stretching whenever we even started to get busy. The day after fucking, I could still feel lingering burning soreness. Not to be gross, but just to be explicit about the type of pain: my ass felt like I had had diarrhea after a really spicy bad chili dinner.
The problem continued even when I was with a patient partner, and I began to think that I had some psychological hang-up that was preventing me from ever being able to "open up" to a guy, or that I had trust issues. Several potential relationships were cut short because I wouldn't bottom for a guy who I liked otherwise. I felt like I was being a tease, since I gave every indication through body language that I wanted to bottom, but couldn't back it up. Following the usual advice, I bought a toy and practiced at home, using different angles and lots of lube, but it still hurt.
Finally the other night I Googled just the right combination of words and found out that some people suffer a burning sensation down there when they come in contact with glycerin, a very common lube ingredient—even in lube specifically made for ass play—and I went out and after some searching found a lube without glycerin, took it for a test drive, and it is a brand new day. Now I guess I'll have to carry around my own specialty lube, but I feel like this is the most amazing sexual discovery of my life so far. In talking to friends who "never bottom," this kind of irritation seems to be the common reason. Although I consider myself fairly well educated about sexual health, I'd never come across this bit of info, and I hope it helps someone!