No, no, no, you are not stupid at all.
I'm sorry if I made you feel that way. I was actually at a training course yesterday and trying (and failing) to be helpful, but I was thinking last night that I had not really managed to express myself well.
"I should have dealt with this long ago and I should have done it very differently."
Well perhaps. But it happened when you were at your lowest and were not in any position to deal with it.
There is another person who should have dealt with this long ago and done it very differently, and that is the person who is responsible for this whole mess, i.e. your husband.
HE also knows what went on, he knows how much this woman bothers you, he knows that you have good reason for that, he knows he has lied to you, and he knows the rest of the story that you don't know.
This is my suggestion for how you approach this:
No supplication, no conciliatory approach, no "please help me out with the truth about this".
You say something along the lines of:
9 years ago you had an affair with X. We have never had an honest conversation about what happened at that time, and I need that conversation to happen now.
You know more about what happened at that time than I do, and I want you to fill me in on the bits that are missing. The future of our relationship depends on you being completely honest with me now.
I'm not making any promises about how I will react to what you tell me. But I can promise you that if you try to fob me off with the bullshit you gave me at the time, or to minimise what happened, my next move will be to visit a solicitor to draw up a separation agreement with a view to divorce.
+++++
The points is you KNOW that he had an affair with this woman.
You know that over a fortnight when you were at home being diagnosed with PND that he was seeking her out to sleep with at night.
You can presume that he was giving her loads about how needy and difficult you were (your PND didn't show up overnight, you must have been showing signs before he left, and even if you hadn't been he would have said those things).
That was what he needed "support" with from her after he came back - how difficult you were and how unhappy he was in his marriage.
These are all things that you KNOW. And even if there was never any sexual contact (and I personally think that is very unlikely given the high number of opportunities for overnight action they've had over the past decade) that was still a complete betrayal of you and your trust in him.