Hello. Adding to this thread a bit later on than the others, so apologies for that. I had the same diagnosis on my second child about four years ago, Downs and Patau. I had very similar feelings as you but knew always as a deep level it was the right descision for me and my family. At the time my DD was 4.
I had the support of a fantastic midwife who helped me through the times, from making my descision to checking in on my after I had the termination. I was 14 weeks and had an induction. if you want more details about that than pm me.
it was hard afterwards, I can't sugar coat that. I had to grieve for the child I was never going to had, to meet and so on. but with the support of my close friends and allowing myself to grieve I got through it. I did feel guilty but soon realised for me the termination was ok and the feeling that it was right for me never left and got me through it.
at my hospital we had the option of the baby being buried / cremated and I attended the service at the crematorium. I'm not religious but the service really helped me as I got to say goodbye. I went alone as I did feel as I was carrying the child it was more my story, event, time to grieve than my DH. this service really really helped and I would strongly advice you doing something of the same nature, write a letter, let a ballon go free or simply look at the sky and say what you need to.
for me, but of course everyone is different, I thought about my baby that I never had (this is what I call him/her in my head - I didnot want to know the sex) everyday nearly for a year, it became easier and less painful and more wistful. 4 years on I still think about him/her but only when I am reminded or come across my scan pictures etc. I will add that I may think more about it than others as I was unable to have any more children due to fertility issues with my DH.
apologies for the lengthy post and the loss of capital letters. in essence, my inner voice that told me it was right for me meant I could have the termination and deal with the resulting emotions. It sounds like you have that too, so listen to yourself as you know best.
take care of yourself and feel free to pm message me, always happy to talk and more importantly, listen.
lots of love