Recall, that description of thinking they are going to die in a moment of happiness is EXACTLY how I felt. Exactly. The thought will still pop in occasionally, like the other day I saw a baby book with a space for first birthday pictures and automatically had this pang of actual grief that ds2 wouldn't make it to then, as though it were inevitable he wouldn't (he is 8 months)... but now although the physical sensation of fear is more or less the same (sharp, cold, all through me, like someone walked on my grave), there is another part of me that has a sort of "Silly Old Bear" response to it and can see it for what it is. This doesn't make it pleasant but my relationship to the feeling is different so I can let it be even if it is quite strong.
I remember a breakthrough in CBT being a session where I suddenly GOT that actually it is normal to feel these intense feelings of fear and almost panic when you consider your children dying, you are supposed to feel as though someone walked on your grave.. but I think I used to react to it as if it was a terrible thing to think, worrying: "why do I think this? why can't I be happy? why do I have to be so maudlin?" and so on and on, not even entirely consciously. I remember when ds2 first smiled, I would feel this joy followed by this feeling that he would die and it was at times physically painful to look at him and then I felt guilty to feel that way!
As well as mindfulness, one of the things that helped was this book based on Acceptance and Commitment Therapy or ACT (said like the word, not as an acronym). It helped me be more realistic and balanced about what sort of parent I am based on looking more dispassionately at the evidence e.g. going through specific incidences in which I was stressed or troubled and looking at what happened logically, but also in terms of setting out the values of who I want to be as a mother...
ACT is about learning to mindfully accept your strong negative emotions, experiences and feelings without needing to react to them or allowing them to determine your course of action unless you choose for them to do so: to feel the fear and do it anyway, so to speak. I spent quite a bit of time having done the work in this book spending time with ds2 where I would give him a baby massage and spend time playing with him being mindful of all the feelings in a non-judgemental way, both the surges of joy and flashes of pain, and I realised that both would arise, peak and fall away and both are just, well, life. Anything you love as purely as a child will also cause you potential pain. We could live without suffering but then we would live without happiness too, they are opposite sides of the same coin so to speak.
None of this was easy, it was actually pretty stressful to go through all these things - it isn't a quick fix. I was lucky to be having CBT weekly with a therapist I liked and felt I could trust and I had CPN visits fortnightly too, these kept me ticking over. In the early days when I listened to mindfulness tapes, I found them incredibly difficult and I felt despairing at times. I couldn't really do it every day as it was overwhelming. Somehow it did sink in over time though, and once I sort of managed to do it a bit more regularly, say 4 times a week of a 20 minute meditation plus a bit of reading about it or listening to books related to the subject on Audible, it started to make changes.
The big change though was going to an actual Mindfulness evening - I go to a drop in class every fortnight and have done a drop in day with others and it made so much more sense. This helped me make the final commitment to daily practice and now I do it every day. If I don't have time for a formal practice, say if the kids are ill or dh has to be out and it just can't be managed, I will try to do mindful tasks e.g. have a cup of tea I am mindful of, or sit with the baby and be mindful for an extended period of time or do the cooking or washing in silence in this way. There is an American book called Momfulness which is very Christian/Western in focus and although I am not religious, many of the "practices" in it are very practical, like ones for washing your baby, or sitting watching your children play or making time to look into their eyes when they are talking to you etc etc. My 3 year old thinks it is hilarious when I go silent and will contrive all sorts of ways of engaging me but I can "look" at it more clearly when I am not talking, weirdly! If I could answer him back I'd probably shout at him to naff off out of the kitchen or some such!
I have also signed on to do the 8 week actual course in April and have been doing an Online course in between - this one, which you can pay for with PayPal.
It definitely requires effort, commitment and perseverance - a bit like training for a sport or something like that. No quick fix but I have definitely found it worth it. Good luck!