I am a 19 year old boy, and I just hate puberty. All day I have thoughts of sexuality and sex. I also hate all this hair on my pubic area and chest. I trim them a lot, but while I am trimming my body hair liquid always discharges -- sometimes with pleasure, sometimes in a small amount. This a huge sin, right? Does this mean I have a sensitive body? Does this means I can't have sex because I am sensitive, thin, lean, and not to mention that I have a penis which is so small? I hate puberty, and I wont have sex right?
You are making conclusions that are not rational.
First, I agree that having to deal with sexual desire is difficult, but then it was difficult to deal with your hunger when you were a child. You don't remember it completely, but your parents remember the days when your hunger drove everything -- but eventually self-control exerted itself. Today, you don't give hunger much thought because your self-control is a habit. God chose to delay when humans have to deal with the desire for sex until after we reach our teenage years. At least this way everything doesn't hit us at once, but once again we have to learn how to control our impulse for sex. "All discipline for the moment seems not to be joyful, but sorrowful; yet to those who have been trained by it, afterwards it yields the peaceful fruit of righteousness" (Hebrews 12:11).
It is not a sin when your body gets aroused and produces pre-ejaculate fluid. It is the normal physical response and it is due to you being naked and handling your genitals. It does not mean you have an overly sensitive body. It means you have a normal body.
The shorter you trim your body hair, the more itchy it becomes. If you prefer to keep your body hair on the short side, that is your own personal preference. However, I think it would do you some good to accept how God made you. If you are a hairy man, there is nothing wrong with that.
"Too small" is a relative term. The normal range for an adult male's erect penis is between 3 and 8 inches. Very likely your own penis falls within that range. Regardless of your penis size, you are able to have sex. What matters is learning how to use what you have and not how big you are down there.