“…doesn't everyone deserve to be happy in our short time here?”
Yes OP, they do, but ‘everyone’ includes the four children whose happiness you are jeopardising with your seedy, loathsome conduct.
You brought your children into the world and inflict this on them? A liar and a cheat for a mother and a risk of a broken home.
You speak of them as though they are getting in the way your happiness - when in fact, it’s you, their mother, the one who should keep them safe from harm, who is getting in the way of their happiness.
If you want to fix this for yourself, you do it by putting your children first. And allowing the other children involved, the chance for their parents to put them first. That means you have to stop acting with such utter selfishness, greed, arrogance and immaturity.
Tell your lover that you have come to your senses and you want to concentrate on fixing your relationship for the sake of your children.
Take it from there.
You will never have a chance to be fulfilled and happy unless you do exactly that. Although of course, your fiancée may just chuck you out of his life anyway and who could blame him.