New to this.. Premature Ejaculation?..

Okay so, I'm not sure if this is premature ejaculation or not.. But this thing happened to me that freaked me out. Me and my girlfriend are both virgins and we've been together for 4 and a half months (She's taking some sort of shots to prevent her from getting cancer since it runs in her family so this delays our sex life for over than 6 months). I'm frankly okay with that since I love her and I don't mind waiting that long. But the thing that scares me the most is that we did some "dry humping" one day and I came after like 3-5 minutes. She was just wearing her panties but I was fully clothed (she did not notice me finish because we had to go 10 minutes after that and I'm not really planing on telling her). Not only that, but she also told me that she's a "nympho".. A light state but still a nympho. That is which scares me the most. If I won't be able to satisfy her, I'm afraid that she might leave me. And to talk about foreplay.. I'm able to go for over 15 hours (tested and proved) but I'm more than sure that it won't be enough, so I'm just worried if I really have premature ejaculation and the thing that gives me the most pleasure is satisfying my partner to the fullest.(Both of us are 18)

I was wondering about the counting thing and the breathing one aswell. Other than that, any advice? Since I still have somewhat over 2 months to shape up... Else I guess I'm fucked..