You sound totally normal to me. I agree with the poster who said that being 15 is part of why you supposedly have PE. Masturbating in the shower is also totally normal... in fact, I'd bet it's the most popular place for guys to jerk off. My experience at your age was that the only downside to masturbating in the shower was that the soap hurt my urethra near the tip of my penis. That resolved itself in about a year, presumably as that area desensitized.

Do you feel like you have PE the second or third time you masturbate in the day? Often orgasms happen quickly when a guy hasn't orgasmed in a while, which at your age is like 12 hours.

Still, it wouldn't hurt to talk to your doctor about it. None of us here appear to be doctors. You might feel embarrassed to talk to your doc, but any decent doctor knows that adolescent masturbation is totally normal.