Congratulations! It's great you want to give breastfeeding another go! It will be worth it!
If your last dd was feeding 'for hours and still not happy' and you felt you were having supply problem it is possible she had a
poor latch (wasn't actually sucking on the breast in the way she needed to).
This would explain why her feeds were so long and why your breasts weren't being stimulated to making the correct amounts of milk.I would recommend you try and get some advice before you give birth and after about what a 'good latch' looks like. Have a look at the video clips on this website:
http://www.thebirthden.com/Newman.html#Video%20ClipsAsk if there are lactation consultants/ breastfeeding counsellors available at your hospital or contact your local La Leche League and ask if there is someone in the local area who can support you.
Your body makes more milk depending on how much your baby feeds. If you supplement or 'top up' your body will make less milk and you may find breastfeeding does not get off the ground. If you want to succeed I would recommend BELIEVING you won't need to give any supplements. Every baby is different and every pregnancy is different. A few weeks in, once your milk supply has been established, if you still feel you really want to give formula then your milk supply will be more secure (but it will still decrease with the amount of formula you give.) It is also believed that if you give a baby a supplement from a bottle when they are very young they may become 'confused' and not be able to suck at the breast properly. Breasts and bottles require very different sucking techniques. That is another reason it's worth trying to avoid supplementing at the beginning. There are also proven health benefits for the baby if you're able to just give them breastmilk.
As for the sleeping, breastmilk is digested more quickly than formula milk. Formula milk contains proteins which are harder for the baby to process and therefore it stays longer in their digestive tracts and they feel fuller. In my experience some breastfed babies sleep through quickly, but not all. Women who choose to mix feed do so for lots of complex reasons. I don't know of anyone personally who has decided to sacrifice the benefits of exclusive breastfeeding just to change a young baby's sleep.
Whatever happens know that breastfeeding will have lots of benefits for your baby. Any breastmilk you give them for however long will be valuable. Do come back if I haven't covered anything.