Think of it as a kind of emotional indigestion. When we get too big a dollop of emotion and we can’t digest it all at once, then it can get stuck, and for some time afterwards we can struggle to digest - or process - the event and the memory.
If this stickiness carries on for some time - months and years, we call it post-traumatic stress disorder - or PTSD. But don’t worry, you’re still in the first few days and weeks, and this is actually quite a common reaction. A post-traumatic stress response is unpleasant but not a disorder.
The hard part is not to avoid the feelings and allow yourself to take time to process and feel what you feel. And from what you’ve said the idea that you’re “over-reacting”, or that others have it worse and therefore you haven’t any right to your feelings - is a problem here.
Telling your feelings to stop, that you shouldn’t feel them, or that you have no right to them is a form of avoidance - by trying to block the feelings from existing. When, as it is in this instance, the aim is to experience, process and let the feelings go, these reactions get in the way.
If you can, gently expose yourself to the feelings. WIth support, from family or close friends, sit in the basement. Approach, don’t avoid. Feel what you feel, cry, scream sob or freak out, but mostly breathe. If it gets too much, then retreat, but gently keep at it.
Allow yourself to talk to those who will listen, as much as you need to, about what happened and how you felt. Often we can feel like we need to talk to others who were there and get the story of what happened straight in our mind, that’s okay too.
But overall, be compassionate and gentle with yourself. There is no competition when it comes to distress and what you felt doesn’t need to be any better or worse the anyone else’s feelings.
It just is, and with time, likely quite quickly if you gently approach, this will become just another bad memory, not a traumatic one.