An obvious question... are you wearing clean shorts every time you ride?

Also, don't discount the hormonal possibility just because you're younger.... as our hormone levels change during our cycles, it does leave our parts somewhat more susceptible to infections and with a lesser ability to heal if they already have one.. so your body may just not have enough change to catch up before it basically deals itself another blow (estrogen levels drop just before your period, and stay low through the first part of your cycle, so if you notice it getting worse around that time, it may be why). Estrogen supplementation may help, and it's something worth asking your doc about if nothing else seems to be helping.

I'm only 28 and I've had to use estrogen to fix things down there (not cycling related, but I was still miserable!).. so it happens