The first time, I was 12, and he was 16. It only lasted for a second because I thought I was going to piss the bed!
The second time, I was 13, and he was 12, but he only had 2 1/2 inches, so I don't think that even counts.
The third time, I was 20, and he was a couple of years older. He had 9 thick inches, and it went it like a breeze. To his credit, he rimmed me, lubed me, and stretched me with his fingers. All the same, had I known that 9 inches should be difficult for my essentially revirginalized ass, it probably wouldn't have worked. My point is that alot of successful and enjoyable anal sex is mental. Lubing, stretching, and experience is one aspect, but mental relaxtion, mental comfort with the situation, and confidence are equally important.
The only other time I ever had significant difficulty was somewhere in the late 80's. I was in a bit of a horny craze, and neither of us had a condom. I was going to let him do me anyway, but my a-hole sealed itself up tighter than Fort Knox. My old 2 1/2 inch friend wouldn't have gotten in there. Some other part of my brain was still thinking, and that inner fear of HIV caused me not to be able to relax (thank goodness), and it gave me an impenetrable hole.
A fear of pain could cause the same difficultly. So, do the lubing, stretching, and condom, but be aware that the rest is mental. You've got to relax your hole from inside your brain. Also, instead of letting your next partner insert, put him on his back, get on from the top, tell him not to thrust until you're ready. You control the extent and speed of the insertion, and all of the initial thrusting. Get it right a time or two and that confidence will take you a long way. Happy humping!