There's no use hanging on to people who are unwilling to stay. Let him go. Let him live the life that he is choosing for himself by leaving you. It may be a great one, but yours will be too. By him choosing to leave, he is opening so many doors for you to meet people who are willing to stay and never let you slip away. Maybe him leaving is the best thing that could happen to you and someone else will come into your life and offer so much more than he ever did.
Make yourself a priority. Focus on you. Do things that make you happy. Eat all of the chocolate you want and binge watch so much Grey's Anatomy that you could become a licensed surgeon yourself. Work on you and give yourself time to heal because you will. You will heal and everything will be okay, but it won't happen overnight. It takes time and thinking about him will hurt for so long that it feels like it will never end, but it will. One day you will reach that point where you feel happiness again and you're happy with who you are and the way your life is unfolding. You will be happy that you let him go. Losing him is not the end of the world. It is just the beginning of all the struggles you will eventually endure and in the end his decision to leave will ultimately make you stronger.
Don't focus on him or what he's doing or whether or not he is missing you as much as you're missing him. Move on. If he chose to leave and didn't fight to keep you then why would you want him in your life anyway? You deserve someone who fights for you and will never let you go to bed angry no matter what it is you're arguing about. You deserve someone who brings you flowers when you're sad and holds the door open for you and enjoys all of your little flaws. Let him go and wait for someone that will keep their promises and never leave you. Wait for someone who truly cares. Wait for someone you deserve.
Never forget the wonderful times you had together though because every happy memory deserves to be cherished. Remember all the date nights and all of the deep talks you shared. Remember the laughs and the way you used to feel when he held your hand in public and kissed your forehead. But realize that trying to force him to stay in your life and care about you when he doesn't anymore will only hurt more than moving on. Let him go, but continue to wish the best for him. Don't harbor any bad feelings toward him because there was a time when you would've done anything for him and he would've done the same for you. It was temporary, but it was good. He deserves to be happy just as much as you do even if the truth of that thought hurts. Use the experiences you had with him to help you grow and utilize the lessons you learned together to make your next relationship a successful one.
Let him go.