Reacurring blood in semen.

I'm 18 Years old. & when I first noticed blood in my semen at 17 I was very scared it might be something severe. I went to my doctor to have an exam, & they told me it should go away in about a month. I also got my prostate checked, just to be on the safe side. & My Urologist told me I was fine.... But months have gone by & sometimes it comes back. I figure it's useless to contact my doctor about it because I'm afraid of being told the same thing as last time. I'm really concerned & I've done all my research.... Is this something I'm going to live with for the rest of my life or is it serious if it keeps coming back. I have so many unanswered questions & it's embaressing to discuss with everyone. Can someone please give me some advice?

-Jason