It probably won't go down well but I'd suggest getting a gratitude journal and start to write in it every day what you're grateful for.
In the end many people follow this same pattern are you right now: "when I have X I'll be happy" - except it never works like that.
When I have a boyfriend I'll be happy
When I have a child I'll be happy
When I have a house/flat/garden I'll be happy
When I have my third or fourth baby, then I'll be happy
But whatever we get, we always want more. Once you have another child, you'll probably want another one.
What is 'happy' anyway? Is your version of 'happy' the only one that counts?
You have two healthy children by the sounds of things. I would be grateful for that blessing and give everything you can to these two beautiful creatures.
Another baby will only divide your time further and if you want the research on it, for every new child that arrives, the other children, particularly the eldest experience an overall decrease in happiness. Children thrive on attention. It's why onlies tend to be the happiest. Again research-based.
Whose happiness is most important? Yours, your DHs, your existing childrens?
I think you have bought into a dream for so long you've forgotten to appreciate everything you already have which is a wonderful man who has looked after your other child and given you a brand new one.
What more do you want? You sound a bit spoilt. I would suggest learning gratitude practices to feel at peace with yourself and with what you have already got.
Happy isn't having more. It's learning to appreciate everything we already have and feeling grateful every day that we get to live on this amazing planet, healthily, surrounded by love, in a safe place.