Would you forgive your BF if he left without saying good bye? And then returning almost 2 years later...?

I would. He has been through hell. After two years he returns which means that he never stopped thinking about you no matter what. Put yourself into his position. Dealing with all the mess he had to deal with was tough and he needed to regain strenght and energy. He wanted to start over new. But he never forgot about you. He didnt want to say goodbye to you because he was afraid of hurting you. He did break your heart. But that wasnt his intention. He just didnt want to see your sorrow and pain of him leaving you. It would have been harder for him to go. He left you a letter and its better than not leaving anything at all. Try and give him a second chance. Work things out by taking one step at a time.