Hi Excile, I'm sorry to hear that - but I think the central problem for you isn't that your W is dating - it's that you are struggling to let go and move forward/on following your divorce.
I hope this will help you accept that both you and your ex wife do now have separate lives apart from the important job of coparenting. I think you have been (perhaps unrealistically) hoping for a reconciliation right after divorce. But I don't believe your ex would have finalised the D had she felt inclined towards this. In the much longer term, who knows?
But, in the short term, please focus on yourself and rebuilding your own life after all that has happened. FWIW, I don't feel dating is a great plan just now - but I would encourage you to start doing some social things or other things you enjoy - and do them in that time when you don't have the kids.
I don't believe it is a problem to potentially be willing to consider a possible R further down the line. However, I do feel it is a problem to wait and hope against hope to the extent that you don't live your life independent of her. Perhaps this happening will help give you the forward momentum to really start letting go and accepting the end of the M.
You may want to google Divorce Recovery Workshop as I notice you're in the UK and people seem to find the workshop really helpful.
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