Hi Mooky, I'm so sorry that you're experiencing this (and to you too, Fred), but I think that you do need to be honest with the GP so that they can do a thorough treatment plan.
I had terrible antenatal depression last pregnancy which started at about 10 weeks. I was petrified that my baby would be removed at birth - doesn't help that we have Police Officers in the family and one day a marked car rolled up outside my door! At 23wks I was admitted into a psychiatric intensive care unit because I felt my body would poison the baby and I needed to cut her out or end my life in a way that would still give her a chance if the emergency services got there fast enough. So it really was quite bad - sorry if this is TMI but I think honesty is the best policy.
I was a voluntary admission - I'd spoken to my GP several times but wasn't really getting anywhere so when it got really bad they sent me directly to A and E where I was met by a specialist MH team. I discharged myself the following day (as soon as I had spoken to a psychiatrist) because the hospital really was awful but it was a means to an end. I was then seen daily in my home by a 'Crisis Home Resolution Team' who were fantastic up until the birth. Ended up being in hospital for three weeks as DD was prem but I was visited in hospital by the Mother and Baby consultant Psychiatrist and by a Community Psychiatric nurse, so was pretty well supported, and this continued at home and in a Surgery after the birth.
When the time was right I also got referred for CBT therapy, which I did for about 18mths. I stopped taking the meds in August and fell pg again in September (happened very quickly, so a bit of a surprise) but this time is completely different and I have felt shit, but in a normal shitty kind of way, and nothing like last time.
It is an awful ordeal, but with the right help and support you will get through it. I took about two years to get over it from start to finish but I have bonded with my daughter and she is the most wonderful and amazing thing that has ever happened to me.
You will find a way to make things work out. Money will be tight but your baby doesn't need brand new things, if he/she is loved then that is all they really need. There are meds that are safe to take during pg and bf and this is now a recognised condition without shame or stigma (in most parts of the health profession - although I would get yourself under the specialised MH umbrella asap to make sure you've got the correct diagnosis and medications). You could be in it for the long haul but there are people on here that will be there for you whenever you need them, and don't be afraid to ask for help!
Good luck with it, sorry for the essay. x