Shift, baby, shift. You regulate your HR and power output with the paddles on your brifter. You adapt your cadence to what feels best for you over a goodly distance. Don't climb at the same cadence you use on the flat. Go 10-15 rpm lower. You're just not shifting enough. Of course I'm assuming you're not riding a fixie or a SS bike. If you are riding a one-gear bike, well that's why people ride them, to vary the workout more.