Sounds like a bump in the road but if you stick with the breastfeeding, you can get past it. Formula is a less than optimal substitute for breastmilk. As long as you are still lactating, you can increase your supply by nursing on demand and there are some teas and supplements out there that can help you to safely increase your supply. YOu only have 5 more moonths of breastfeeding and then you won't even have to think about substituting your breastmilk. As far as the mastitis is concerned, I had that too. I took homeopathic Belladonna for it and it cleared up within a few days. IF your breasts are hot, you are feverish, it's a good remedy. I would bet that his teeth are irritating you because your breasts are sensitive from the infection, but once that clears up your nipples should toughen up enough that it won't bother you anymore. You can use lanolin in the meantime to alleviate the discomfort.
Breastfeeding isn't always this blissfully delightful bonding experience, but most of the time it is. Any mom who has done it for any extended period of time will tell you that there are difficult periods, but we get through them with perseverence, patience and understanding.
If you want to talk you can always write back to me or there is lots of support in your community by finding your local La Leche League chapter.