Aww Highland you poor thing, its not nice at all. For what its worth, I had baby blues on and off for about 6 weeks with DD1, and luckily only about 2 weeks this time. In my experience though, it very much 'felt' hormonal, crying for no reason, then happy as Larry 5 minutes later, then dreadful guilt later on. I really did feel like I was up and down like a rollercoaster. I didnt ever feel 'complete and utter despair' or or wanting to 'run away into a dark corner'. This does sound quite severe and I hope you are getting some of the highs as well as the lows.
I think the difference this time round has been that I've HAD to get on with things normally as have an almost 4yo DD, so no time to dwell at all.
Also I have a much better support network this time, and I seem to remember you are out in the sticks a bit, so adult company is probably in short supply.
The only advice I can really give is that when your DH goes back to work, you get plenty of rest, dont try to do everything, also get outside for a walk or some other adult company EVERY SINGLE DAY or you will go crazy! They are called sanity breaks for a reason...
Give it another week and speak to your HV again if no improvement, it is not good for you to feel this way day in and day out.
Please dont worry about off-loading on here, thats what we're all here for!! 
Hope you feel happier soon, and keep in touch. (((((Hugs)))))