I'm glad you are where you are surrounded by love. It will help you regroup yourself.
Children are very intuitive, they've probably already picked up on the fact that you are sad. There is nothing wrong with telling them (maybe not the 2 year old) that Mummy is sad right now but will be better soon. It helps them know that they are not the cause of your sadness since little ones sometimes think that way.
You've had 2 bad breakups one right after the other. No wonder you are devastated. But you will heal in time. That's just the nature of things. There's nothing inherently wrong with grieving relationships, even short term ones, as long as you try to put them in perspective with the rest of your life. You have many, many years ahead of you. In time this will be just a blip on your life's radar. Concentrate on what a good person you are; a good mum, daughter, sister, friend. And the good things you have in your life; children, family, and friends.
Please, tell me you didn't leave him alone to stay in your home. He needs to be gone. There is no room for him there. You need to start rebuilding happy memories there of you, your children, and those who do love you.