He cheated but why do I feel awful having to divorce?

I've actually discovered more and the affair has been going on for 2 years, not a year as previously thought. He got my 41 year-old friend pregnant.

I never wanted to be in this place. I think if it wasn't for the affair child, I would've tried working it out. Then again, maybe not. I don't know. I really can't think of anything awful I ever did. During his affair (when I was unaware), he changed so much, was giving me attitude, became obsessed with being more fit and losing weight, neglected me for intimacy, was suddenly working "extra shifts" and we started arguing more.

My parents and other relatives don't know the reason I'm divorcing. I feel awful having to do this. As if I've done something wrong or carrying some type of huge shame on my head. Why?

Updates
10 mo
I want to add this detail that feels even worse to me:
They had unprotected sex because he didn't think she would still get pregnant at that age.
He cheated but why do I feel awful having to divorce?
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