newmomma hello am on the may postnatal thred,only just seen this post 
I can only respond by saying that my first DD was exactly like this and that I felt the same as you. I genuinely cried every day. I was at the GP every other week and was told she was simply "grumpy". She cried all of the time.
I remember being at a weaning workshop and the HV taking me aside and asking what was wrong with my DD. I had just fed her so she was not hungry (also breast fed) and she was just crying. I had actually learnt to almost ignore it by then (I know ignore sounds awful but nothing I ever did calmed her down). HV told me it was not normal behaviour for a baby and that the doctor was stupid to tell me she was just grumpy. To be honest, 2.5 years later I am not sure who was right. She remains bloody hard work!! However I now love her to bits, but can see why she was a difficult baby even now.
I used to cry and find any opportunity to leave the house, picking up the take away etc.
We started to mix feed about 4 months. Dr Brown bottles are brilliant, as is colief - breaks down the lactose in formula. Now saying you should switch to formula but I found introducing a couple of bottles a day gave me a break. SOunds awful but she became someone elses problem for a few hours
as she cried regardless of formula or breastfeeding.
Looking back I can see it was all tummy problems and feel sorry for the poor mite, but at the time, the crying drove me crazy. I dont think I had PND. I think I was down and anxious because of her behaviour, not buce versa.
It really was awfulbut it got better. Infant gaviscon helped a bit, colief helped a bit - neither made the crying go away but they helped her to nap in the day a bit.
Time helped a lot. By the time she was weaned she was an angel baby.
Have you read the baby whisperer? Some babies are classed as touchy and need lots of time out. I think my DD remains like that.If she is fed, burped and tired, a couple of minutes crying in a dark room might help her sleep? I know its very hard and even saying that now I remember my DD would not have settled butyou cantry it.
Sorry you having a hard time, its not you, certain babies are sent to test us I think. It will be fine soon ...