Re: Giving it another go after failing last time around.
"Toughening up" your nipples with a towel is an old wives' tale--it won't do anything but make your nipples hurt. I nursed my DD throughout my pregnancy with DS, and his latch made me hurt anyway--those little newborn mouths are small and they don't latch very well at first.
Those soothing gel pads are nice for easing pain and soreness--I think Gerber makes some, and there are some others called Soothies. Also, exposing your nipples to air helps, too.
A shield can cause supply problems, but that's usually later on, not in the first weeks. A shield can help at first, but some babies get very attached to them and it can be hard to wean them off them.
What I would do is line up lots of help afterwards, both for breastfeeding and for yourself and family. Get some childcare or playdates planned so you and baby can snuggle skin-to-skin and snooze together while your toddler is playing happily and safely. Have freezer meals made, or plan for lots of takeout, or see if family/friends/church will bring some meals in. Maybe hire a postpartum doula to support you with cooking, cleaning, emotional support, etc. in the postpartum time. Start going to LLL meetings now, while you're pregnant. Find an IBCLC and have her contact information ready so you can get reliable, good-quality help when you need it. LCs are not all equally trained and qualified.
Catherine, tandem nursing mama to Preschooler Girl 9/08 and Toddler Boy 3/11
Breastfeeding for 4 years and counting!