Unhealthy erection. Help is welcomed.

Hi,  I am 23 years old and I have no trouble getting an erection during masturbation or foreplay. I can get really hard. However during sex I cannot maintain a really hard erection as I do during foreplay and masturbation.I can't put it through. At some point I feel getting softer. Also sex does not feel very stimulating to my penis. I feel that I would never reach an orgasm no matter how long I continue. And if I attempt to stop, to change position or because penis was accidentally removed my erection is easily lost and without removing the condom it is hard to get it back.Along with I have Little curved penis and I don't smoke but occasionally drink. I am really confused cause I know there is nothing wrong with me physically and mentally. I am a relaxed type of guy. no worries. i enjoy my life. I don't understand this. Any advice and help is welcomed. Thank you!