Elly, I'm sorry to hear this. As others have said, it is so hard to hear those words and of course you are upset.
I think, as with so many of our sitches - we need to let go in order to try and hold on. I'm not sure what you not agreeing to sign the D means - but if it would mean resisting to the D, I think it would be best to work towards acceptance of it. Work towards losing any fear of D. He wants us to D, this is what may happen. It isn't what I want and I have told him that. But it is what he wants and I accept that. That's been my approach anyway.
Have you considered what part MLC may be playing in your sitch? Your H would be the right age for it. And the POW/EA? in the picture (running behaviours) and 'it's not you - it's me' remind me of my sitch. Has your H demonstrated other potential MLC traits? I have found the Hearts Blessing site helpful on MLC a treasure trove of articles & Q+As.
Whilst it's horrible, at least you know where he is at right now - he wants to D. But as you say, nothing else has changed really. Not in DB terms. Him wanting a D is not the end of the road - not by any means. The best you can do is recover from the convo (which I also think you handled well) dust yourself off and get back on the pony...there is always hope until you decide otherwise.
Take care xx
T 13 M 7 Me 48 H 46 SS 15 BD 7.14 PA D final 5.16 (H filed)
We receive & we lose, and must try to achieve gratitude & embrace with whole hearts whatever of life that remains after the losses - Dubus