Why don't you give the SNS or lact-aid a chance? They will eliminate the problem of nipple preference, as well as they'll help you increase your supply. I was in a situation very similar to yours, and it's thanks to that SNS that I'm still breastfeeding - and supplementing, but as I said, the volume of formula my baby is getting is lower than it was a month ago. It's under control now, whereas before I felt I didn't know how to stop the wheel, my baby was getting more formula and less breast each day, I felt tired and sad, it was awful.
Don't give up, you'll feel better!