Why can't i get an erection when my girlfriend is around?

When we first started dating, I got hard all the time around her. Seeing her hot face behind those glasses turned me on. Then we went to 2nd and 3rd base, and i still got hard. After coming on her glasses a few times and putting my penis between her breasts, i dont get hard anymore. But when my best friend comes over to my house to play video games, i get an erection like crazy. When i make out with my girlfriend and rub her breasts, im lucky to get half an erection. But when my best friend pats me on the back, my penis zips right up. and its not that im gay or anything, cause i know im not. even sometimes other girls hug me and i get an erection. so its not just around my best friend, its just not around my gf. and recently she gave me a bj and i got semi erect, but couldnt get harder. she asked if i was alright and i said yes. so she kept going but after a while she stopped. I just want to know why this is happening to me. and a few times my step sister and brother come in my room to play video games and i get an erection. and also when i try to watch porn, it takes me like 5 minutes to get hard. whats wrong with me?