Aimsmum - the think about anger is that it can be a bit like a security blanket- losing it is troublesome
I think what may be behind this is that while your ex was being a twat it was easy not to see the man you loved in him. Now you've had a glimpse of the person he was, it is only natural to be sad that he was lost to you and your DD for so long
I would say to try and keep your communication with him as open as possible- no matter what happens, it will be best for your DD to see mummy and daddy being nice to each other even although they don't live together any more