Ok, first some questions.
How is your DD's weight gain? Is she making lots of dirty and wet nappies?
People do manage to feed on only one side. And, from what I know, mums always have a "better" boob and a "worse" boob. Both my boys preferred my left side, and DS2 still does.
My left boob is normally a bit bigger. When I am bf, the size difference is greater, but from what I remember, they were back to being (nearly) the same size, between babies.
I wouldn't bother with the expressing, it's not necessary, and it's just annoying you. You can keep offering the left boob first, to try to balance things out, if you like, but please don't get stressed about that.
What do you mean by hurting underneath your boob while feeding? Inside? Shooting pains? Or what? When do they come? Do you get them on both sides? Do you ever get them when you're not feeding?
Congratulations on the birth of your DD, by the way. You're doing really well with breastfeeding. The first six weeks or so always does suck (ha ha) with tiny babies.