I’m at 22 year old male, I have had a good sex life my whole life since age 17 I’ve slept with over 20 girls since high school and outside of it. I never had a problem with getting Hard soon as I look at them it’ll be at easy but now I’m in the longest relationship I been in which is 3 years. it’s always good sex like 4 times a week but sometimes I have periods I can’t get hard enough for sex or it’ll go away soon as it time to penetrate and I’ll cum quick I don’t know what’s wrong I think it’s just in my head I can’t stop thinking about it I don’t want to be “broken” because I love my gf and totally turned on by her but I know it works because when alone I can keep a fully hard erection for long I want as I even about sex but when it’s time for the real action my nerves kick in like I haven’t been here before I’m lost 
Hi, I’m 24, something similar happened to me, I’ve had sex a lot of time with different girls, but I just ended a 4 years relationship and I when I decided have sex with other girl, I just couldn’t stop thinking about something going wrong, and then I couldn’t get hard, but it’s all psychological, just try to relax, and stop thinking about it and it will work