It sounds like you might have developed depression and may be stuck there. It happens sometimes with grief, and it is more common when there are unanswerable questions about a death or situation. It is not easy to let go when you have unanswered questions. Your mind can get stuck on wanting to know those answers and staying back with the grief and PTSD, leaving you unable to move forward.
You may always feel grief over what happened to your son. But you should not have to live with suffering that feels relentless. What may help at this point if you have not tried it, is medications. Some mild anti depressants can help change your brain chemistry enough to trigger more recovery. Also, in therapy, working on changing your thought process. Accepting that you will not have answers to your son's death and working to change the thoughts that keep you feeling distressed can help you move on. For example, when those thoughts come up, you can develop a saying such as, "I know this hurts and I may never have answers, but that is ok. I can let go". Also, thoughts about your son being at peace can help as well. Repeating that to yourself may take a while, but it can help you accept and move on.
Thank you so much for the rating and bonus. I really appreciate it.
I am sorry for your loss. If I can help in any other way, please let me know.