Unable to maintain erection. For this.

Hi, I am 28 years old male recently married to a 27 year old girl. My wife and me both are virgin. While having intercourse, I get an erection very fast and very tight but it lasts only for 10 seconds and I can literally see my penis going flaccid. I can feel the blood going out of my penis. It has happened multiple times . I can manually stimulate my penis and get a hard erection but the erection goes away as soon as I remove my hand from my penis. Sometimes , my penis is hard for a long time but not hard enough to penetrate into my virgin wife. I have masturbated for past 5-6 years almost on a daily basis but never paid  enough attention that as soon as I stop manual stimulation, I lose erection. I love my wife a lot and she also loves me. She is very supportive towards me and I am under no performance pressure. I am not fat, and I don't have blood pressure or diabetes. Can you please guide as to what can be the cause and cure for this. Thanks in advance