I think a lot of it is psychological. A LOT. As Redrhodie said, shifting your focus off your speed and onto technique or (is it possible?) enjoyment of the task works wonders. After first doing a little stand and climb, I shift down to a spinnier gear, look at my front wheel, and start counting breaths. Often a song will come into my head, or I'll let my thoughts wander. I don't look at the other riders. A guy might hammer by me but by the top, because I'm not pooped, I can often pass other riders who were kicking butt at the bottom.

Slow and steady. Relax your shoulders. Move a little forward. Sing a song. Recite the alpahabet. Count your breaths. Ease up a bit. You'll find it much easier to breathe!

(p.s. this is all stuff I learned right here on TE!