How do I fix this problem?

Me (18M) and my girlfriend (18F) sometimes have problems during sex. Sometimes it’s great and I finish in like 5 mins, sometimes I can’t finish, it’s like my penis just struggles to stay hard and I don’t know why. She’s not the problem but she thinks she is and whatever I tell her, she doesn’t believe me and thinks she’s the problem, this is probably our 8th time having sex but it’s just soul destroying and embarrassing, idk what the problem is and wether I should seek professional help or not, someone please tell me what to do.