I just want to echo what Branleuse says.
I do feel hugely for you, OP, and it is a big, and good step, starting to talk about it. It's perfectly understandable that you feel sad about it. Hindsight changes everything, and it puts decisions that you made a long time ago into a whole different light.
I know this might sound unsympathetic, and I really don't mean it to be, but: I just can't seem to stop thinking if the what ifs and wishing I could go back and tell myself it would be ok.
Do you have a time machine? Though if you do, I'd like to borrow it!
I have these conversations with my OH all the time, as he is one to dwell on things that he could have or should have done. But as much as we would like it to, unfortunately we just can't go back and change things and so, when it really boils down to it, thinking about what should have happened doesn't achieve anything. It's like the concept that 'comparison is the thief of joy', only instead of wishing you had a house like your friend has, you are wishing you can change the past.
The strangest things can make us suddenly become very upset about things in the past. I know what I said above it very simplistic, because termination regret is also about grieving for what could have been, and grief is certainly not something you can shake yourself out of.
I'd also recommend counselling at the moment. You have beautiful DC and, most importantly, you made the decision that you felt you had to 12 years ago. No one has a crystal ball that can tell you how things will work out, so we can only make decisions based on the facts and feelings in front of us at that time. But talking to someone experienced might really help you to put these feelings aside and not let them affect your day to day life with your DC (not to ignore them; these things are part of us and we should never try to just ignore them).
I hope you don't think I'm being harsh, I really don't mean to be.
for you for feeling so down about it at the moment.