I am still breastfeeding my son who is 2.5. I never had any doubt that I would breastfeed. For me, formula was not an option. I only gained about 18 lbs during pregnancy and within a week I had lost it all and then some -and am now 10 lbs lighter than I was pre-pregnancy. I am pancreatic sufficient so I'm sure that helps me keep from getting too thin but I think that if you are committed then keeping weight on shouldn't be an issue. It will just take a little bit extra work and some vigilance about your diet. I've never had a problem putting on weight but for the first year or so I did have to make sure I ate enough to keep me where I was.
I had a doula for my birth who was also a lactation consultant. She helped me A LOT and we did need to use a nipple shield for the first week or so because he didn't have a good latch and I was in a lot of pain. During that time I could see why women quit and switch to formula but again, that was never an option for me (self imposed) and after that week it got a lot better. I clearly remember the first time that nursing him didn't hurt. Nursing my son is one of the experiences of early motherhood that I will treasure the most. I still love it and my son still needs it so we'll keep going till he wants to stop. And nursing is great for soothing an older baby - plus its a sure fire way to knock them out for a nap or bedtime. When my son get sick, sometimes the only thing he'll do is nurse - he'll refuse any food or water and I've never been concerned about that because I know he gets what he needs from me. He's never been a big eater so even at a year I'm sure that 90% of his diet was still milk. At 18 months milk was probably about 60-70% of his diet. Now at 2.5 he just nurses to sleep and when he wakes up unless he's teething or sick then he nurses for comfort more through the day.
Bonnie - I just wanted to comment on what you said about pumping when you need a break - just keep in mind that if your breasts are used to being emptied every 2 hours and then you decide to have your DH feed your baby a bottle at feeding time, it will be another 2 hours till you empty your breasts again and let me tell you, it gets very uncomfortable if you miss a feeding. I am convinced I have a small storage capacity in my breasts so my son nursed every 2 hours till he was well over 8 months old (he would go 3-4 hours at night). So missing a feeding was actually pretty painful for me. So even if you feed a bottle of expressed milk, you will still want to empty your breasts to avoid a plugged duct or engorgement.
Lastly - I remember reading an article that said that even in severely malnourished mothers (not talking about CF) breastmilk remains pretty consistent in composition. So I wouldn't worry about your baby not getting enough of anything. And taking extra vitamins just to boost the breastmilk doesn't work either because it will still stay consistent.
Do get the name of a lactation consultant before you deliver to work with post delivery and see if there's a support group you can meet with once a week. It's totally awesome but not always easy. Isaac has been such a sweet baby which has made it easier I would imagine.