I am absolutely disgusted by most of the comments here.

OP made a mistake, a massive one. She cheated, that's wrong. Completely understandable that her boyfriend had trouble with it.

But pretending to be a in a relationship for 18 years, sharing intimate moments together, all the while having this plan of breaking up with her once their son turned 18, is exceptionally deceiving and worse than cheating. OP could have spend the last 18 years with somebody who actually loved her, who she could have had more kids with. Instead, she was with somebody who faked the whole thing. If the boyfriend had made clear is was going to be a co-parenting thing, then fine. But he didn't. He had sex with her, pretended to be in a relationship with her. For 18 years.

That's next level cold and diabolical.

Those praising this man for doing his duty have just because OP cheated and that means she deserves this, are absolutely wrong. Cheating once doesn't mean you deserve to be lied to and lead on for 18 fucking years. This isn't just a few months while the boyfriend made his mind up. If be couldn't live with OP's cheating, he should have said so, so they could properly move on. He decided to put on an act every day.