There is something wrong with my penis and all that stuff PLEASE HELP!!?

ALRIGHT. Im 15 yrs old and I think there is something wrong with my penis, maybe my prostate. I sometimes have difficulty peeing and somwtimes get pains around the area. I havent told anyone about the difficulty, even when I try it is too embarrasing. I went to the urologist, he said he didnt see anything wrong, and I also had a sonogram of my testicles, everything turned out fine. But Im afraid. The doc said masturbation wasnt harmful in anyway really, so I doubt it is that. Buuuut, when I started masturbating I sometimes closed the hole in my penis so the semen wouldnt come out. I was young, I didnt know it was suppossed to be like that. Could that have been harmful???