How do I deal with my mom having an open affair?

If your mom has an open affair, and that is affecting you / your friendships/grades, you have to talk to her, and tell her the way she feels, any mom will do the best for her child, don't tell family members , it may worsen it, or hurt your mom's feelings. If she thinks that her affair is more important than you , you can talk to a social worker at school (for example) but BE SURE that any affair is temporary and if you feel that your mom loves someone else more than you or your brothers and sisters or your father , well that's not true, growing ups do mistakes too, and they all wake up one day and realizing what they did and that their families are all what matter.

Without communication, there can be no peace. Talking to her and telling her that it makes you feel uncomfortable is always the best answer.

Speak with your mother about how you feel, from there you can both figure something out, affairs are hard for a spouce but even harder if there's children involved.

I'll be sad of course but it's her freedom. If she's happy then I'll be happy. There's no use working things out in a family long broken. It's time that I support her happiness.

Having a heart to heart conversation with your mother may help you get your feelings out and on the table. It's quite possible that your mother has no idea that you are not a fan of her having an open affair. It can be difficult to so many and known by so few.

Tell her that you do not accept her actions. Tell her that it is wrong and that she should not be surprised of the aftermath.

You try to look at her as the human being that she in fact is. And like all human beings we make mistakes, err in our ways, and sometimes even when we want to avoid hurting others or feeling a certain way it isnt something we can quite help. Another way to look at it would be to understand that maybe mom is having a hard time right now and needs you by her side more than ever. Even moms make mistakes/suffer demons of their own. I would definitely try discussing it with her after you've taken all that into consideration and have given it lots of thought.