He made a choice. Even if you take in all the factors that could have been the reason why, he still made a choice. At the end of the day, he made that damn choice and that’s to let go and find someone else.
Now if you truly love him, you should also make a choice. Make a choice to set yourself free. Free from the thoughts that poison your mind and soul. Free from the memories that yes, will sometimes make you smile but mostly make you cry. Free from the grasp of your past.
Just decide. Just choose. Nobody is asking you to move on quickly. But you have to make a choice. Then you can take slow deliberate steps. That’s all you need right now. Make a choice as much as he did.