What are the best ways TO MAKE A GUY REGRET LEAVING YOU?

Don't waste time trying to make him regret it. If he wants to be a kid, let him. You have a chance to be a kid with him, or mature and grow up some more :-) Spend some time with your friends and just have some fun with them. Then try to meet someone else when you're ready. Trust me, don't wait for him to grow up...he may never. Meet a man that wants you and wants the same things you want without wanting to be a teenager again.