sexual, physical,mental problem?

Ok. So im a healthy good looking guy, 24 years old. A year and a half ago I got out of a long relationship. 4 years and engaged. I continued sleeping with her for up to a year after the split. Then we stopped. Now There’s another girl I’m really into, we been hanging out/kinda dating for 6 months. She finally, after the longest wait of my life with a girl, wanted to have sex. So the first time she was wasted and lied there like a sack of rocks. I wasn’t into it so of course I couldn’t get an erection. Normally if a smoking girls nude in front of me that’s not a problem. So she was embarrassed, n didn’t wanna try again. A month later (last night) we go at it again. I was dead tired from snowboarding all day but I figured it would be a good way to end the night. So we get into it, I go down her pants. I got an erection, she pulls me on top. Then…. nothing. It just went away. Im worried this could be a problem. The girl no longer wants to see me, Im embarrassed by myself. She thinks I still like my ex n don’t wanna sleep w another girl. She also said It could be that I’m nervous since its a new girl, which Im a lil worried about performance after 6 months. but that’s never effected me before. I don’t know if im used to having sex with my ex. Which was the standard fuck around, foreplay, I get head, then sex, where as this girl doesn’t give head. idk… does anyone have any suggestions as to what may be my problem, whether physical exhaustion, or a sexual problem, or could it be a mental problem? Im so lost right now n feel horrible. Thanks

No It is neither sexual nor physical and nor mental..it is psychological..
Tip: second time do not write this much long texts so that you’ll get more answers dude =)!