Is it normal to not care about an affair?

My Mum had an affair on my dad for eight years, I found out when I was 10 (I'm now 17) through a drunken phone call she had with him one night. Basically, it all came out last year because he threatened to tell my Dad and so my Mum ended it with the man and told my Dad everything instead, and basically 'sorted it out'. (I assume the reason being she wanted to stay with her kids, and in her home, not because she really loves my dad). Anyway my dad forgave her and they're still married.
My dad has been driven absolutely crazy because of it. He breaks down in tears and is broken in every way possible. He didn't deserve an affair, so obviously I'm not condoning what my Mum did, but is it normal to not care? I still love my Mum just much, even though my trust in her has been tainted a bit. I want them to get a divorce instead of awkwardly pretending they still love eachother.
I mean, I found out when I was 10 and didn't tell anyone the whole time - is that normal? When I think about it I feel crazy, letting my Dad go through that and bottling it up to myself, and still loving my Mum when I basically knew everything because it was clearly laid out in front of my eyes. (thanks Mum.)

Should I resent her more? Should I have taken action? And how can my Dad stay with that? It's not like my Mum is begging at his feet for forgiveness. She's mostly blaming him my Dad for what she did. Is this normal..?