What do I do in this situation?

Me and this girl I like have been talking for about two months now. We are both 20 and we have been seeing each other a lot lately. We haven’t had sex yet but last night, things got physical but my body completely shut down and I couldn’t get an erection. When we were outside and making out I could get hard but I didn’t really feel horny. When we got in the car things just became flat for me.

A couple hours before I saw her I watched porn which I know probably ruined my chances to get hard with her. After I realized it wasn’t working, I told her that I just couldn’t get in the mental headspace to get an erection. She seemed disappointed which makes sense because it seemed she was extremely horny but I just wasn’t there. This was very embarrassing for me but this was not the first time this has happened with a girl.

Why does this keep happening to me? It’s like I can’t get excited when it comes to sex to even get hard. It’s the next day and things have been awkward with her. How can I fix this situation?