quick read and post
firstly, sounds like your GP dismissed you a bit quickly
PND can manifest up to a year or more after birth
feeling low is excacerabted by lack of sleep
you need to try to break things down into manageable chunks
go back to the GP and ask for help ! or see a different GP within the practice
being tearful, problems sleeping and feeling low, not looking forward to things, mood swings are all part and parcel of depression, and again with lack of sleep , will feel worse
get in touch with local surestart / homestart and get some help and support
if you can afford help in the house , get some !
see about addressing DDs sleep issues
PND is horrible, it is insidious and it is wearing, and it can fill you full of guilt for feeling like that about your life and oyur children
it is an illness, same as any other and needs treatment
can your DH go with you to the doctors? is your HV decent and approachable?
I have been there, and it sucks, but it can and does get better
and we are all here for you