OP thanks for sharing your story.
I have NC for this thread.
I just want to add a perspective from the point of view of the child of a marriage that turned out to be riddled with infidelity.
I am now in my 30s and my parents are still together in their late 60s and have a busy social and work life.
my mum found out about my dad's affairs when we were still children (all primary age) but it was kept secret from us until we were adults.
However, they couldn't conceal the fighting, shouting, bitterness, long-term hatred. I remember how loving they were towards each other when I was very small - but I am the eldest, and my younger siblings don't ever remember them really seeming to love each other.
I think it's a mistake to think that it's always better for your children if you stay together. As an adult, now, I wish my mum had kicked him out permanently when she found out. I have an OK relationship with both of my parents independently, but it has caused a lot of problems down the line for everyone in the family, directly and indirectly,
It would have been really tough for my mum to go it alone, no doubt, but it wasn't a picnic living through all of that either, for any of us. Family holidays were a nightmare
In a sense my parents are happy together now, but not really. I don't think my mum's ever really truly forgiven him deep down, and that comes out in a lot of unexpected ways.
So OP when your mum says "you have to be able to tell your children you did all you could", that doesn't necessarily mean staying together...