He's not with her, it happened September 2011 which I suppose is something. She thought it was all dead and buried but they were still in touch via email and text even now she's in the UK we are in Australia. He said she knew the score and he was never leaving me for her, almost wish she was the love of his life and worth all this shit.
What helped me was to try to realise that the need to know is just a form of trying to control ones own feelings of rejection and abandonment ( IYSWIM) - he didn't actually leave.
If you analyse it, what extra information would it give you, whatever he did. The 'why' is interesting though. At the time people told me that I was over giving on who he was and why he did what he did, at the expense of myself.
In fact I think it's a stage you just go through. Thinking about him and his motivations actually helped me see what he did as rather sad, but at the same time I was in no doubt about the effect it had has on me.
If you can't get beyond this stage Mos, read Pittman on philanderers. Because that is the type of infidelity your h does, for sure.
It's not picking at scabs: it's the need to know in the (possibly vain) hope that you will come to understand what is currently beyond comprehension. Sometimes imaginings are worse than knowledge, and sometimes the shock of discovery has a protective effect (IYSWIM) when making further discoveries.
You do however need to think about whether you really want to know the answer before you ask. And consider if the response you get is likely to have any truth in it.
As you seem to have decided on separation, then I think you should be finding ways to make it happen and quickly. It might be difficult (and cost him money), but that is simply part of the consequences of his choices.