*My ex had MH issues and the last few years we were together, he told me that sleeping with me was like being with a prostitute as he thought I was really loose.
....My ex was pretty horrible because he developed a delusional disorder where he was convinced i was cheating on him with someone who had 'stretched' me. It was all in his mind but logic aside, it has still left awful emotional scars*.
Delusional disorder ... Yet you're taking what he said as valid/truthful, and letting it affect your sex life with your nice, new partner.
Have you had counselling, perhaps you should have more.
You're not particularly loose, you're not anything. Your ex is insane.
Your bed bf's relative smallness (Vs the average male) is a separate issue. It's up to you two to work out how to have enjoyable sex .. if you're not having enjoyable sex.
You being paranoid, based on the statements of a lunatic, that you're loose and trying to do kegels throughout is totally unnecessary and must be affecting your enjoyment (?)