I always wonder if I had PND with my first child to some extent. I was unhappy and avoided doing things out of the house much, snappy at then DH. If I did it seemed to fade eventually so I think I had it mildly but it all felt like much harder work than it should and the self doubt was extreme. I was shocked by this as I was a nanny to a young baby in the years before i married and had children myself and was always very maternal and looked after several children and coped very well with that.
For me it was the pressure of getting it right and I over-worried so much about so little.
When I think of my subsequent pregnancies -where I didn't feel depressed, my whole attitude was different. I was coping and laid back and I didn't have a fear of it all going wrong.
If we say I did have some form of PND with DD1 then when I had it it made me totally incapable of basic stuff, and also made me utterly paranoid about my baby dying, keeping me up all night checking her. To me, that was extreme over anxiousness and a sign of depression.
I'm a little concerned about one of the replies here, which I am sure was well meaning, but please don't add the worry of your DH giving up supporting you. A good DH - which is what you describe - will support you through this whatever the cause and no matter how much of an arse it makes you.
If you were to run away, it would be dreadful but you wouldn't lose everything, the decision makers in these things (I mean people like GPs or Social Services) would recognise th ecry for help. I'm not saying run away, I'm just trying to point out, without any disrespect to the poster, that you needn't become even more fearful of your frightening feelings - that will only add to your anxiety.
Now that you've recognised you have a need, however that need is described (depression, PND, anxiety, tiredness, etc.) you can move to the next stage and address the problem by seeking help.
Whether you tell your DH, your GP, your mum or your HV first is up to you but do tell someone - just enough to make them aware that this is becoming an unmanageable problem for you. You don't need to bare your soul or reveal your most dark thoughts, you just need to say you need help to cope because your coping ability is failing you more than days that it is fully functioning!
Think about what would help, if you know. At work, I say to people 'if tomorrow was a magical new day where everything was near perfect, what would be different?' This can help you determine what your needs are.
You may need practical help, emotional support or medication for a while, and none of these types of help have a severity score attached. You could be on your knees with desperation and all it would take was someone to help with chores and similarly you could be mildly depressed and yet medication would be the solution. Don't feel bad about identifying your needs. Everyone has them, we need friends, drugs, coffee, chocolate because life is hard and life as a parent is double the responsibility.