We had an almost identical situation in 2020. Our daughters condition wasn't guaranteed to be terminal in the womb, but if she survived she would have needed lots of immediate surgery and then subsequent surgeries all through her life, with no guarantees, and very poor potential quality of life even if everything went perfectly.
We decided that we didn't ever want her to feel one second of pain, so horrendous as the decision was for us, it was the only choice we felt we had. You do anything for your children, regardless of how hard it is for you - and that's what makes you an amazing mummy. All your little one will know is the love and coziness of you, and that's an amazing thing.
I'd agree with everyone with regards to saying take your time and allow yourself to feel what you need to emotionally, but would also say please be aware of your body and push with doctors if you don't feel right afterwards. I didn't, and it was a mistake. Ended up needing surgery further down the line because I just assumed the weird feelings/bleeding were normal. Post-birth care isn't renown, in this situation its often even worse, so if you don't feel as you're expecting to, make your voice heard or have someone ready to advocate for you.
Also, please don't be surprised if your milk comes in. I didn't expect it and it threw me completely. I took the drugs to stop it but a little still came in and sat in total shock with maternity pads in my bra while my mum was dispatched to find breast pads from somewhere (which was tricky at midnight, bless her!)
You'll get through this OP. Take time in chunks - ten minutes at a time, then twenty, then an hour, two, three, half a day, and you'll get there xx