I can easily gain an erection but cannot maintain it

Well, dehydration is not good – and while it may seem unconnected, it can have an effect on the penis.  So the first order of business (for your overall health as well as penis health) is to make sure you get hydrated.  Beyond that, the fact that you have smegma all over your equipment indicates that you may need to do a better job of taking care of your equipment; penis health can have an effect on erections.  Wash your penis daily (including under the foreskin) but use a gentle cleanser – and make sure the laundry detergent you use is also not going to affect the penis. Finally, add a daily rub of a good penis vitamin cream (health professionals recommend Man1 Man Oil) into your regimen.  Look for one with L-arginine, which can help with penis blood flow – and blood flow is important for good penis health.  Good luck!