First of all, Kitten, you should be immensely proud of yourself for sticking with breastfeeding in spite of your stitches, cracked nipples and frankly unsupportive sounding female relations! I do hope it is getting easier for you now.
I'd like to reiterate the point made by quite a few mums that you really need to get your latching on checked, once that is right any pain should go straight away and cracking should disappear really quickly. Breastmilk contains wonderful antiseptics and really helps the healing process. After feeding, don't dry your nipples, just rub a bit of breastmilk over them and allow them to dry naturally. I used this technique and the occasional dab of vaseline and had no problems - but then I did have a lot of help with getting the latch right.
The advice about laying down and feeding is also great, it allows you both to get some sleep. However, by now your milk should be fully in (gold-top quality!) if your latch is right. The 21 day thing is just the sort of mis-information that is all too common.
Just remember, you know your baby best out of anyone in the world and you will know if things are right. Put those miserable relations onto cooking, cleaning and shopping duty and do whatever you need to do to feel you are doing the best for yourself and your baby (sorry, daddy does come last for a while!)
Good sources of support if your MW/HV is useless are the baby cafes which are now springing up across the country www.thebabycafe.org Not only are they great for support, help with technique etc but you also get to sit down and have a cuppa (maybe even a hot one!) and meet other new mums. Definitely one for when you can get out and about.
Take care xxx