I actually will say this to my ex (that's the first time I've called him 'ex' instead of 'husband' I think!) but I kind of wanted to rehearse it here, hope no one minds!
Thanks for your letter tonight, it's given me some closure, which is what I badly needed. Even when I sent you that message yesterday, I was 99% sure that you'd tell me it was over for good, but I needed you to say the words so that I had no room left for doubt.
If I've read it correctly, it sounds like you still care about me as a friend but don't have any romantic feelings for me any more. That's better than some people have in this situation, and I'm feeling pretty happy with that! It'll take a while for my romantic feelings for you to go away, but rest assured, they will, and I'll move on.
I'm glad we can still be friends. It's still a relatively short time since you left, but I'm proud of how far we've managed to progress in that time. I'd love us to be in a position where we can be in each other's company with no uneasiness at all, but I realise that's going to take time.