When we have uncertainty and serious insecurity in ourselves, that's when we go back and second guess things and relook at our decisions. It's an ego thing to want to have our ex miserable without us, like a competition to prove we are happy without them and that we were the best thing they ever had.
They had you and they want to continue to have that power and control over you while they do their own thing and find somebody else. To keep that cushion around and when they start to feel like they lose that control, they do panic.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.