Very cold or very hot showers (and especially the combination) stimulate nerve endings and blood flow and will not help relieve your sexual tension. Yes a cold shower will shrink your testicles but so will an erection. Your better off with a mildly warm shower or turning on a 24 hour news network.
At summer camp there was a rumor that saltpeter was being put into the food to keep the guys from being able to get erections. I don’t think that worked too well either (if the cooks were even doing it, that is).
Seriously, don’t sweat it man. When I was 17, I was having sex on a regular basis and still spending considerable time with the ever-so-lovely Rosie Palm. It just comes (pun unintended) with the territory. Just accept the fact that you are currently as horny as you will ever be and that it is all downhill from here (it isn’t but tell yourself that and you’ll feel better)!