I'm so sorry to hear how tough your story has been. Mine is rather different and I don't profess to have the answers, but hopefully hearing that someone else is trying to deal with similar issues will at least help you feel not alone.
Fortunately I had my DD, now nearly 10, easily. We had a tough first year with her in and out of hospital, but health wise all is fine now. Due to that and the subsequent, but unrelated, spells of depression my DH had, we decided not to have another child. Well, we both agreed during the first year, then I changed my mind, but due to his illness, he didn't. By the time DD was 5, I was longing for another but found it hard to even discuss it given what DH was going through on and off. However, it brewed within me and whilst it seemed that everyone around me was having child no.2 or 3, I eventually brokedown and explained that 'I was simply wasn't done' and that whilst I rationally understood how lucky I was to have one beautiful child, I wanted to be someone else's mum too and provide a sibling. After much discussion, we went down the ICSI route as in the intervening period, DD also had a medical issue which resulted in secondary infertility. It was a 'one time only' deal, which going into, I knew one of us wanted it to work, and one didn't. You can imagine the pressure and negative emotion......so when it failed, I was distraught and he was relieved. To his credit, he did ask if I wanted to do another round but emotionally it was too soon and raw. Three years later, I am still grieving for my loss (that is what it feels like) and almost every day, I think about only having one and the implications. After having some coaching / counselling, I've learnt to focus on the positives and am gradually healing. The really difficult parts are when my DD, who is a highly sensitive and deep thinking child, reacts to situations emotionally which I just know would have been helped my having a sibling around. The guilt then, is horrendous.
But, little by little, I am learning to accept things as they are. I watch the news and see horrors unfold and am grateful for what I have. We have a comfortable life, travel and do lovely things together as a unit of 3, plus our pets are expanding as substitute children!!! I suppose all I can say, is that some days will continue to be painful, but others are full of joy. That's life as they say.
I genuinely wish you well in your journey and hope you find peace.