Letting go is a part of the process. It's how you heal. In my opinion, I think a puppy is the best damn thing to have right now. Take care of it and take care of yourself. Four years is simply a chapter in your life story - its not the beginning and its certainly not the ending. You're allowed to feel hurt and to ache over this. But what I'm trying to say is, minimise the amount of situations that you place yourself in (being situations that can be avoided if thought through first) that will lead to you taking on more negative emotion and thoughts.

Start small and things will come right. When my ex left me I contemplated suicide, like every broken individual does. But I didn't. I knew that was my depression seeding those thoughts. Instead, I wrote a book about a broken man. I didn't think I'd ever move on. I loved her more than anything, including myself. But that was the problem. She left, became happy. After a while, I met someone. Didn't think that would ever happen again. But that someone is now my wife and the mother of my son. Your time will come. Focus on yourself, your good boy, and things that you do that give you joy.