Hi WSD you do sound as if you are heading back into depression. PND can reoccur very easily in the first couple of years after having dc. Fortunately I have only had one occasion when I felt like doing something stupid but didn't but it did give me and DH a real shock and I think it makes you realise how serious PND can be.
I would never ever contemplate anything like that normally. I did manage (just) to claw my way back to some semblance of normality without ADs and it certainly didn't last as long as the previous fully blown PND after DD2's birth, so hopefully this might be a temporary hiccup for you too....but I think you need to rely on any other forms of help you can get if you have them.
Talk to friends openly and have a good cry, talk to family, let anyone willing to babysit and if you don't have this advertise for a babysitter to give you an evening off every now and then. Try to exercise as this is meant to help and music can be quite beneficial too as long as its not too maudlin.
Go back to your GPs and maybe enquire about some counselling, do you have any unresolved issues in your past? If so this may be helpful. I don't know what else to suggest but I hope you start to feel a bit better. It is truly horrible to feel so low that you are even contemplating ways of ending it.
From your own experience what has helped previously?
I do know that recently when DH has not been working such long hours I have felt so much better but unfortunately his job requires long hours and it drags me down again...Have even started to consider advertisng for an au-pair just to help with day to day things and obviously childacre when I work.
Keep chatting to us MNers too as I think it can be a really good outlet to let off steam and vent your concerns.
Just had one more idea which I read somewhere. It can help depression to do good turns for others, not sure why but I did try to do this for a while and it did seem to help. Maybe it's the feeling you get from knowing you have helped someone else...Must start doing this again.
Please post soon to let us know how you are feeling.