hi, thanks for writing..i have understood your problems... you are masturbating excessively...because of that your sexual glands which contain the semen is exhausted...these glands are like factory, they make semen at a same pace daily, but when u remove semen repeatedly then there is lack of enough semen to be ejaculated, so your semen is watery and yellowish.. dont worry, when u reduce the frequency of masturbation to 2 to 3 times in a week, your quality of semen will become normal.. because u are thinking that u have developed big sexual problem, the anxiety arising from that thinking is making u weak and multiple pains in your body.. all other symptoms are psychological.. premature ejaculation is common when a person masturbates, it will become normal when u will have sex with a woman.. the penis size is genetically determined, like negros have biggest penis and south east asians have smallest,,the size doesnt matter.. the sensitive part of vagina for sex is only lower one third which is less than 2 inches..so dont worry..dont compare yourself with porn stars...they are professionals and are of difference racial origin so genetic works in their big penis... do not believe in any myths about sex, prevailing in the local,community... i hope this advice will be helpful to you..best luck...