You sound a lot like me, except I'm small but not super light. One of the classic rides here is to go up Mt. Lemmon- a sustained climb of about 23 miles mostly 5% but ranging between 4-8%. Everyone passes me. One day I counted 60 people who passed me.

I tend to sit and ride in one of the lowest gears (I have 11-28 on the back), but now that I'm getting some coaching, I am supposed to do standing intervals on certain rides. One of the activities has 15 minutes of standing intervals during the ride. I am not able to do 5 minutes at a time, but I could do 1-3 minutes standing (which surprised me).

My coach also gave me this suggestion for shorter hills. On the hill, downshift once, and then once again, but instead of shifting down a third time, go back into a harder gear and stand to complete the hill (or a portion of it). She said that the reason you see pros doing this on big climbs the Tour de France etc is to use different muscles and allow other muscles to rest, not necessarily to attack. Also she wanted me to concentrate on pulling up as well as pushing down in my pedal strokes.

I totally agree with Marni about smiling and relaxing your face. I read that here when I first started riding, and applied on a hill that I ride frequently that gets up to about 10%. In fact, my husband put that as the motto on my Road Id when he ordered it for me - "smile on the hills"