Radical acceptance is a bit tricky, everyone ends up with their own understanding of it.
The way it makes sense to me is this:
Mental suffering happens when we fight with our reality. When we can accept that our current moment, as shitty as it might be, cannot be struggled against, fought with, or changed - that's radical acceptance.
You aren't making peace with it, or deciding it's ok to feel as bad as you do - you are simply saying that it's what your reality is, right now. You can't change it right now. You will co-exist with it. And eventually something will change. Doesn't even mean you have to believe it's going to get better - you just need to know that things change.
Applied to your situation - you feel wrecked after trauma therapy, worse than when you started, and you are afraid, overwhelmed - you wish it wasn't happening the way it's happening. The suffering is a result of the tension between the reality and the wishing.
Accept that you are wrecked. You're tired, depressed, afraid - that's just what it is, for now. Not forever, but for right now. It is what it is, it's really shitty, and there's no changing it right now.
Oddly, when it clicks - it's a profound relief. Just not fighting against those feelings, not wishing they were different, can make big chunks of suffering and stress vanish.
I'm sure others have their own ways to describe it, this is just my way. I don't think I'd be alive still without it, though.