grallagh I am so sorry that you are going through such a difficult time.
I have never posted in this topic before - I read it as I have one DS (2.3) and DH is adament that we are not having any more .
I really think that you need to repost within the Mental Health or Pregnancy topics - www.mumsnet.com/Talk/feeling_depressed or www.mumsnet.com/Talk/pregnancy
"there are so many messages about why a one child family is such a positive family unit and echoes my own feelings entirely" - this is why you need to move from this section of Mumsnet now. Yes of course there are positives to a one child family - but majority of Mumsnet users will have more than one child and will therefore be able to relate to issues surrounding going from 1 child to 2.
You are much less likely to find people here who are going through what you are going through - simply because the vast majority of people who look at this topic only have one child.
I think that you need to print your post off and urgently speak to your GP (and midwife/hospital doctor) - these two parts of your post:
"It's making me sick with anxiety just thinking about it"
"DH and family members think it's chronic anxiety and ante natal depression and that it'll all be ok once this gruelling pregnancy is over but I'm not at all convinced."
are making me worried about you .
Ante natal depression is a real and serious (but treatable) mental condition. Your DH and family members are not qualified to help you with this.
Please do a google search for antenatal depression so that you can reassure yourself that other suffer from this and can recover.
A couple of links here and here.
"Has anyone else felt so negative like this during a pregnancy and it turned out ok?"
I believe the answer to you question is YES. (I have no direct experience but suffered mild anxiety whilst PG and prob undiagnosed PND and through extensive internet searching have come across similar scenarios etc).
You might find this thread thought provoking and revealing.
Have you considered that you may be sufferng from Post Traumatic Stress Disorder following your traumatic birth and early weeks with your DS?
Do you have enough information about your elective CS - this should be a million miles from your emergency CS.
There are plenty of 40+ mums on Mumsnet - they have a thread in chat here.
Something which I have found useful in controlling anxiety is hypnosis (I did a Hypnobirthing course) - I am a big fan of self hypnosis downloads.
There is one called Finding Happiness in Motherhood.
On a practical note do you have pre-school lined up for your DS? He is eligble for free sessions the term after he turns 3.
Please go and see your GP and do not get fobbed off - there are anti depressants which you can take whilst pregnant.
Well done for such an honest post and please don't beat yourself up for not going with your instinct and TTC #2. Antenatal depression can strike regardless of planned or surpise PGs.