When I had mine, she had been in nicu for a few days then lost 12% of her birth weight so I had to combi feed: formula, expressing and breast feeding. So I can totally relate to how stressful this must be for you.
I expressed every 3 hours for 30 mins on each side (crappy hospital pump) then I got a mothercare one when we got home and I only needed to express for 10-15 mins. This produced around 120mls. For the first 3 weeks I found breastfeeding painful and stressful. However once she had surpassed her birth weight I spent one afternoon doing skin to skin and whenever she was hungry I would latch her on and she would suck a little, it would hurt, I would stop. We went on like this for about 3 hours, then the next day she sucked a lot more and it hurt a lot less. After 4 days I wasn't feeding her expressed milk or formula anymore. I took her to get weighed and she'd put on weight. That gave me the confidence I needed to exclusively breastfeed and its been pretty straightforward ever since. I do get worried when her appetite seems to be insatiable before a growth spurt but that only increases my supply.
Keep at it chick and it'll pay off. But if it doesn't, it doesn't. You've been through a lot so don't put any unnecessary pressure on yourself about breastfeeding. So long as your baby is gaining weight and is healthy that's all that matters!