If you once lived without him, you will find a way to do so again.
Stop speaking to him.
I know that it can be hard to put down the phone. It is even harder to no longer talk to them, especially if you haven't gotten the closure you need. This step is absolutely crucial because you need to be away from the person that you once loved. It is only with that separation that you will learn to be without them.
Take time to reflect on the relationship.
There is likely to be a reason that you guys broke up and it's important that you remember why. When you're lonely, you will start to regret the decision. You may think there is something left to do and you want to find a solution.
NO. Think about it and remember what made you want to leave.
Spend time with your family and friends.
Surround yourself with people that love you and see that you still have many people on your corner. Regardless of what happens, you will always have them and that may be enough to pull you out your slump.
Do not date someone immediately after.
You need time to yourself. You need to learn to be on your own and then think about finding someone new. If you reflect on your relationship, find the things you are looking for in the next person, you can then move on.
Remember who you are.
You don't need anyone, especially not someone who will not complete you. Love yourself and from there, I guarantee you will find the right one.
Bonus: eat ice cream.
This can be done in between or during all the steps, it definitely helps.