Erection loss

Patient: Hi, I am a male of 19 years old. I just started to be active in my sex life. Nonetheless, I have not masturbated before, I don’t really know what an orgasm feels like. I do sometimes achieve an erection, but when My girlfriend and I try penetration my erection goes down after a short moment. I do not know if the cause is the feeling itself. If it is just something weird to experiment . I really want to maintain my erection.What are my options?

Symptoms: erection loss when penetration