I agree you probably need to see a doctor. A question. On a night when you jack off and cum, and you don't get it on your underwear, do you still have those dry patches?
I'm not sure it isn't a wet dream. Sometimes they are a little different maybe mixed with precum. I don't know. I just know that my wet dreams were much thicker than when I jacked off. At my age I don't have wet dreams anymore. Precious memories, how they linger.
Still, you might have a bacteria or something there that needs treatment. Failure to take care of it might cause problems down the road. Don't delay. See a doctor. You are 18. The hardest part is talking to your parents about needing to see a doctor. There is such a thing as telling half the truth as in it burns like hell when I pee.
Once in the doctor's office you can tell it all. Do your best to see the doctor without parents. it will be easier to freely discuss this.
Remember that your urologist is going to tell you to jack off 5 times a week. There is absolutely no need to feel embarassed around the doc over this. If anything he will "fuss' a litte because you don't jack off enough.