Why do you still feel drunk the day after you drink?

Slow metabolism. Everyone has a different rate at which they metabolize alcohol & other substances. If you have a slower metabolic rate, you might feel effects for a longer period of time. Also, depending upon what medications you might be taking, alcohol consumption might multiply its effects (or vice versa). Bottom line, while there appears to benefit from moderate consumption, we don't recommend getting drunk.
Alcohol hurts sleep. I notice that you are eighteen years old so i hope your frequency of alcohol use is minimal. Your brain is still growing and developing! also, you should be aware that the sleep that alcohol produces is not as restful. Some think that it is because the slow wave sleep and delta waves that are the most restorative are negatively impacted in the context of alcohol intoxication.