An ultimatum will not work. You have to let this relationship run it’s course, it will take up to two years. It will either fizzle out, which I suspect it will, small children will be a challenge, or it will grow more serious.
You must let go and act as if he will never return. I know you are devastated but the very best thing you can do right now is to keep detaching and be the best Kirsty you can be.
That way, whenever he thinks of you, he will be reminded of your dignity and strength, if nothing else.
