Are you putting a lot of weight on your hands/gripping tightly?

I'm no expert either, but I do have experiences to share: Way back when I used to borrow bikes that were always too big for me, I found that it would help take a lot of strain off of my hands and shoulders if I thought about keeping the weight off my hands. You want just enough weight on your hands to balance and steer. The rest of your upper body should be supported by your core -- your abs and your back muscles. So I would kind of pull in my stomach, like trying to get on a tight pair of pants, but not so much that I couldn't still breathe comfortably.

Maybe try doing a search about foot numbness -- I think there have been other threads about that.