Don't know if this helps, but as a total bottom, I have on occasion switched. The guys I am normally drawn to differ from the guys my cock gets rockhard to switch with and cum in. Prefer mature guys, beefy, but to switch and cum in the guy, my cocks type is slim and younger, no older than 35. I also found that it was the mechanics of sex, so not only his weight but height as well. Not too sloppy. And receptive too. Also cum more when fucking missionary. Boring I know, but as a bottom, not had much practice at trying other positions. It's about finding the right body weight/height ratio and position for you, so plenty of practice like much of the other advice given. First time I shot in a guy I was so surprised I (almost) cried. I was 30 and had been having (consensual) sex for 18 years. So don't panic about it, it will happen.