hi.. i can understand your concerns.. u are about 17 years old and u must be masturbating from the age of 13 years..actually masturbation does not cause any physical or sexual problems, but these kind of misbeliefs are very prevalent in community. masturbation is not a bad habit.. i have interpreted your low sex strength as a erectile problem or premature ejaculation as per your question. that can happen because when person masturbate the speed of stroke is very high and mostly there is some fear of being caught, so person is always in a hurry. when u feel for the first time that u have lost erection early or you ejaculate within 1 or 2 minutes then u started worrying that u might have developed any sexual problem and started blaming your masturbation as the cause of it. these worries continuously bother your mind and u are nor able to concentrate on anything even on your sexual feelings..so these all sexology related problems are not actually problems, they are just reflection of your worries.. another thing when you masturbate repeatedly with a frequency of once or twice a day, then there is no enough semen in prostatic gland and seminal vesicles, so instead of thick whitish semen u ejaculate only liquid straw like semen..that is a normal phenomenon..once u decrease your masturbation frequency the semen becomes thick again, try to reduce your frequency of masturbation.. i hope this advice will be helpful to you..best luck..