I do actually trust her.... maybe mistakenly....
You surely can't say this with a straight face.
If someone I was with for nine years shagged someone two days after we split up, then we got back together, split up again and they started seeing the person they shagged again then yes I would be incredibly upset and hurt.
I would also realise that's an unsalvageable relationship that would from that point on be toxic due to the hurt and trust issues, so I wouldn't get back with them.
You've chosen to get back with her, you've tried to make it work but your trust issues (which I can understand to an extent) are now making you behave in a way that is very clearly becoming controlling and paranoid.
Again, I understand why you may feel paranoid but that doesn't mean it's right to start being controlling and accusatory. What it does mean is that you need to accept too much has happened, you've tried, the relationship isn't going to work long term and you need to break up and focus on coparenting.
Otherwise the resentment and paranoia will worsen, you'll become controlling and the dynamic will become toxic which will ultimate damage your sons wellbeing too.
You tried, it didn't work, time to end it.