my partner and I have had sex for over 3 years, i've been masterbating ever since I could remember, probably 5 years tops and now whenever me and my partner do it, for the past few months my penis would lose it's erection and sometimes i can't last long. I used to last pretty long and do it all the time, but i think me masterbating too much is the problem? I also think about losing my erection during intercourse. what could be the problem?