Just wanted to post to offer some hope to all those of you who are experiencing similar and looking for support.
A few weeks ago I was feeling absolutely wretched: waking up every morning crying, filled with rage that was causing relationship problems, suffering from stress and anxiety that seemed completely disproportionate to the situations I was experiencing and totally unable to enjoy life with my 3yo and 7mo DDs.
I realised I had a real problem when I took the Edinborough test. I went to the GP, she put me on ADs (paroxitine which I am able to take while BF) and referred me to a counsellor. The ADs kicked in quickly and the counsellor offered me a course of CBT which is really helping me to understand why I get so stressed and work out ways of breaking the cycle. I still get very anxious but life is definitely much better and I no longer feel like the crappest mum in the world 
I was so afraid of getting PND (depressed before and had a rough pregnancy so knew I was susceptable) but really, with the right treatment and support, it isn't so bad. Much better to face it head on than deny your feelings and hope it will get better by itself.
As you were.