Someone has mentioned Beacon House org uk upthread, and they have a video on their website about helping someone overcome childhood trauma using EMDR.
OP, in the short term, and this is going to sound ridiculous, but simply by saying to yourself a few times "there is no threat there is no danger" it may help. At the bottom I link a video which is relevant though I haven't checked the credentials it is quite basic info and very easy to understand how things can by pass the thinking brain. I have found that by instructing the brain that there is no threat can help.
If you are suffering terribly then talk to your GP. In terms of medication, something like beta blockers can help turn off the panic giving you space to think things out for yourself, and you would normally be only on them for a couple of months, they work immediately and don't affect hormones or "you". I think it basically does turn off the fight/flight but you do have to deal with everything else. You could also talk to a clinical psychologist who specialises in trauma or like a pp has said you may be lucky with a referral. It sounds as though you have a lot to unpick so long term you might find that helpful.
As far as can understand the science says "name it to tame it" so you overcome the trauma by being able to pinpoint how you feel about an event and your brain can then selfsoothe - that is my very basic understanding though there is a lot of research and a clinical psychologist with relevant expertise would be able to explain better.
Another short term thing that helps provide release is EFT tapping - google and you will find a video where someone explains the technique - it is really good for releasing things - you can "say" what you want while doing it and do it over and over.
The very most effective thing I have ever done to help with this sort of thing is to write it down, exactly what happened with all the detail you can including how you felt, and then read through it - this helps your brain process it and gives you sights. And going through all your emotions - how it made you angry, how it made you afraid, how it made you sad, how it made you feel sorrow. It is hard with a 4 year old to look after but if you have moments to yourself when he is asleep that might be a good time.
This video explains in a really easy way basically what is happening which may help give you understand the role of your thinking brain. I am not sure what the guy's credentials are but it is just basic info and I found it helpful at the start of trying to understand what I could do. But if you feel it is overwhelming you please do seek professional help/talk to your GP. Dan Siegel has some good videos on recovering from trauma, and speaks about "tame it to name it" on youtube.
Lastly, long long walks, really good for both you and your 4 year old in terms of reconnecting the brain, helping the thinking part take control, releasing feel good endorphins.
At some point consider how things may have affected your 4 year old.