OK, number 1 - breastfeeding doesn't cause your breasts to sag. If anything does, it's pregnancy - so nothing you can do about that now! A lot of people feel like breastfeeding made it worse, because they get used to the plump and full feeling breasts that you get in the first couple of moths before milk supply regulates but in reality the ligaments (coopers ligaments) that hold breasts up are not affected by breastfeeding.
Regarding flat nipples, sometimes it cause cause problems, often it doesn't. the difference is in the severity of the flatness/inversion. Do your nipples stretch/stick out when pulled or stimulated? or are they tethered so that they are always flat no matter what you do? Sometimes if nipples don't stick out at all, the baby's suck reflex doesn't get triggered, and this is where a nipple shield can really help. If you decide to use a shield, make sure someone knowledgeable helps you get the right size and shows you how to attach it. However some women with flat nipples (like my mum) feed with no problems - this is because the baby should not be attaching to the nipple itself, they should be grabbing a mouthful of breast tissue around the nipple so that the nipple itself is positioned far back in its mouth.
Most people find the first three weeks of feeding difficult and a lot of people wont feel like they've cracked it until past 6 weeks. How easy it is depends on a number of factors - physical ones like your nipples and the baby's mouth, and emotional ones like whether your expectations of motherhood and babies are realistic or whether you have people around you supporting and empowering you.
If breastfeeding is something you a really committed to do, it may help to educate yourself about what is normal baby behaviour. So many mums think they aren't making enough milk when in reality their baby is just being a baby (wanting to feed often, wanting holding all the time etc.). Find out about growth spurts and supply and demand. If you are a reader, books like 'The womanly Art of Breastfeeding' are great. If that's not how you like to learn, try to find out if there are any support groups near you. Meeting other mums who are successfully breastfeeding can help you set realistic expectations, and when the baby arrives you will already have a support network to turn to.
I've just seen you are in Leeds and I know there are breastfeeding support groups. Search facebook for 'Leeds Bosom Buddies' and you'll find a page which should give you information about all the groups currently operating in the area. Also, your midwife should have told you about PBB - Preperation for Birth and Beyond - classes that run for 10 weeks and are free to all pregnant mums. All midwives in Leeds are supposed to be telling all their mums about these and helping them sign up to them so pester her for info!
