I am 21yr old male and I have been dating my gf for a little over a year and 3 months. We have been having sex since roughly 8 months into the relationship. We have more recently started experimenting with the girl-on-top position and reverse cowgirl. I have no problems keeping an erection when I am on top of her but when she gets on top of me unless she is grinding me really hard and fast I lose my erection pretty quickly. Is this relatively normal? Is there anything I can do besides maybe not masturbate for a few days prior? We usually have long foreplay/oral sessions up to 2 hours before diving into intercourse. Could this also be another factor? Thanks for the advice ahead of time.