Lonelymum - I can totally sympathise about not wanting to invite anyone back because the house is a tip. So is mine - and I really mean that. It is one of the biggest things to get me down. But I have to say, that on the occasions when I do clean, I feel so much better for it.
Why not set yourself the goal of tidying one room to sit in and invite 1 or 2 mums over for coffee. You do not need to prepare anything fancy - a few biscuits are fine.
What ages are all your children?
I remember forcing myself to go to the mother and toddler group which I actually hated for months
Then I decided to go on the committee (which I had avoided because I thought they were a clique)
Anyway, I then discovered they were actually all nice women (mostly), all trying to have some kind of life outwith constant childcare.
I also got involved in the school parents association. I have some very long term friends now from those early days.
Please try out your GP. I know mine socially but he has been excellent when I have gone to see him and cried in his consulting room because I have felt so low. I never needed medication but I felt so much better after talking to him. I know that he keeps our discussions confidential because that is part of being a doctor.
Enrol in your night class anyway - your husband might have difficulty getting another job and you may find you are still in the same situation when the classes start. You can always cancel it if need be. Don't leave your life on hold.
This post kind of jumps about a bit but I hope you can see some sense in it. You are not alone - many of us have been there. You can make things better - you just need a bit of support. Friends do not happen overnight but you may find that you actually click with someone at the school or playgroup. Give it a go.