Do you have a low enough gear for the climbs you are doing? Climbing, as you have discovered, is very different than flat roads. I'm lighter now but most of my riding has been in the 220# range and most of my climbing has been with lighter, fitter riders. There is a lot to learn. Take your time and figure out what works for you on long climbing days. It may be quite different than what works for the skinny mountain goats.