Weaning makes your hormones go CRAZY! I was 5 months pregnant with #2 when i weaned my son (he was 17 months) and I was a hormonal train wreck for about 2 weeks after his last nursing session. Your stresses with your 3 year old are probably adding a lot of fuel to the fire too, but there is hope! All of my nieces and nephews (12 of them) were awful 3 year olds that turned into downright-delightful 4+ year olds!
These years with small children are so incredibly draining on us mamas. Do what you can to relax and do something for YOU at every chance you get. And to the poster below who asked why you're weaning: I think the answer could very well be because she does not want to breastfeed anymore. 15 months is a long time to have someone else dictate what your body is used for, and I think its awesome that you are working gradually to wean him even though it sounds like it's much harder on you than it is on him. A happy mama is the best mama!