The regular partner thing is true(for me anyways). With all my partners I’ve always been able to go hours(5.5 hours was my longest before I just got completely frustrated and told her it wasn’t working for me…they take it much better than I would).
My current girlfriend, and only longtime partner, and I were the same. We had sex every week or two and it would always be marathon sessions. But when we started having sex more frequently our sessions were greatly reduced(not quick, but a comfortable time).
I’m actually glad it happened this way, I know exactly what I like with my partner, and she knows what she likes. I know what will get me off, and in turn, what to avoid doing. Might be more help than a curse.