Thanks for pointing that one out. I've always wondered what the purpose of snorkels was whenever I would see some people swim some laps with them. I myself had the same problem like yours, of holding my breath in and not exhaling enough, and this would eventually get me tired much sooner than I should be.
My comment on the snorkels though, specially for the newbie swimmer trying to improve his stroke - with the snorkel on, one would be unable to practice the proper head turn for rhythmic breathing.
In my case, I addressed it by using the same TI approach in imprinting a particular skill/movement in a stroke - by using breathing as a focal point for a particular set in my workout (eg, 500m swim, focus only on breathing). Eventually, my breathing became instinctive, while still being able to practice the full stroke :)