I have been through exactly this OP. I'm so sorry that you are there now and I completely understand the shock that you are feeling.
The diagnosis we received was pretty much identical to yours although at 18.5 weeks and picked up by the Triple test (not sure this is still done). I received a phone call at work saying I'd been booked a scan as a 'caution' and call me a pessimist but I knew straight away as there is spina bifida in my family.
Because the scan clearly showed such severe issues, we decided not to wait for a second opinion and we did terminate. I can honestly say it was the worst period of my life, going through shock and grief as well as worrying how my friends and family would react (stupid I know but I don't think there is a 'normal' way to react / deal with this). Having a family member living with SB meant even more guilt but our baby's prognosis was much more serious than family member, with hydrocephalus being as serious as the non closure.
All I'll say is it is a horrific position to be in and while lots of people will have opinions, you need to do what is right for you. If you'd told me I'd have a termination before that point I'd have said you were crazy. Our baby would have had zero quality of life and I still know that we made the right decision.
One mistake I made is shutting myself off from everyone. Until the termination had been done, I didn't tell a soul with the exception of DH which was a mistake. I was at my lowest ebb and really should have talked. The 10 year anniversary was last week and to the rest of the world, I look like I have it all now but I regularly still break down when I'm alone and I think it's because the emotions are still bottled up. Ive still never really spoken about it to anyone other than very matter of factly and while I can write and even speak quite well about the factual experience, I struggle hugely with the emotional side - but that's me with lots of things to be honest.
I went on to have a healthy daughter who is almost 9 now so it was pretty quick afterwards but I think this was part of my coping mechanism.
Happy to answer any questions you may have and again, I'm so sorry that you are in this position. Xx