Morning plumchum. It's no wonder you've become unwell, PTSD sounds about right, I can't think of anything more terrifying than thinking you might lose your child.
I can relate, until recently, I was a lone parent to my four dc. They used to go to their dads every weekend from a Friday to Monday morning. A couple of years ago, I had a lung cancer scare and whilst I was waiting on results, I remember thinking, if something happened to me Friday night, I would just lay there until Monday morning and my kids would be the ones to find me.
It's a horrendous feeling. Sertraline helps with PTSD as well and I definitely think CBT would help if you could get some.
I tried to think about it as rationally as I could. I could text my mother before I went to sleep every night and text her when I woke up first thing. That way if she didn't get the morning text she would know something was wrong and come over.
We all know in life that unexpected things can happen at any time. However it's incredibly unlikely for a healthy person.
And as you say, you are both on the right meds, things can and will, only improve.