Emily, why do you keep thinking your weight has anything to do with it? You weigh more than the bike. The bike will go where you go. You go where your head and eyes go.

Sand is tricky for anyone, but light riders keeping up his/her speed should float through it better than a heavier rider. Smooth stroke, good cadence, and ski the bike through.

Sand is a fact 'o life here in the west.

You may just need to think more about moving/balancing your weight over front or rear tire more depending on the condition. In sand, you should really just be using the front tire as steering control so only as much weight on it to steer. Tires slide in the sand, period. But a little more weight over the back could keep it hooked up and propelling you forward.