What Causes Premature Ejaculation During Intercourse?

Hello doctor,I am 26 years old and I had my first sexual intercourse in 4 weeks ago.the first time I performed for 3-5 minutes and attained the ejaculation but was not able to get elected anymore.I tried three times after but penetration is still a problem.I can masturbate and keep aroused for 15-30 minutes before ejaculation.But when it comes to have sex,my sex lose it firmity while I am trying to penetrate into my partner s sex.Please tell me what I can do to perform normally. Thankfully Jack