Erectile Dysfunction. This issue?

Hi. I am a healthy male with no existing medical condition. I recently got married and having issues with my sexual life. My erection does not last more than 30 to 40 seconds or maximum 4 to 5 strokes. I have done sex only in missionary position. This does not happen when I masturbate. I am able to last much longer like about 3 minutes; however, during the actual sexual intercourse i can last only for seconds or max 4 - 5 strokes and I ejaculate. I'm not sure, this might be caused due to excessive masturbation in the early years. I am really worried and unhappy about this. I have recently ordered tadaflo tablets to improve this. Is it safe to take tadaflo? Also what should be the dosage or frequency of taking it? Could you give me any other advise to overcome this issue?