Hi there - my DS was the same in that he much preferred my left breast and shunned the right from about 20 weeks - it was also suggested to me to try the rugby hold but he wasn't having any of that trickery! I tried to ensure that I fed him from both breasts at each feed, he would always empty the left one.
It may help you if when he's really hungry, offer him the right breast first, particularly in the night when he's sleepy.
We did end up with quite a few 'green poo' nappies because he was getting a lot of foremilk from the right breast and not finishing it. So if you've got that going on, don't worry too much - just try to make sure he's getting some 'full fat' the majority of the time.
I too did some pumping so as to release the pressure and hardness - I never enjoyed pumping, I found it very uncomfortable, but I figured it was best to keep my supply up in case he changed his preference again!
My theory on this is that babies do tend to prefer the left side as they are next to your heart and can hear and feel your heart beat, just as when they were in the womb. You may also find it's because you are comfier and more relaxed holding him to your left breast, particularly if you are right-handed; and if you're more comfortable, so is he.
As it turned out for me, by the time he was 6 months and we entered the world of weaning, he would not feed from the right side whatsover and I kept becoming engorged and developed a couple of bouts of mastitis, so please keep an eye out for this. Any red patches or severe tenderness, around the breast or armpit could indicate the beginnings of mastitis, the earlier you spot it, the better. Keep a couple of savoy cabbage leavses in the freezer on standby.
Eventually the supply in my right breast settled down and it shrunk - to be fair I did have to spend the summer with a baby sock shoved down my bra to even myself up, but now he's 20 months and just having a little comfort suckle before he goes to sleep, I've a matching pair again, thank goodness!
Please try not to worry - you're doing marvellously well and your DD is too!Just go with the flow......
Sorry to ramble - HTH x