I'm so sorry that you are put in the middle of this situation and are hurting.

Unfortunately, you can't fix your parent's marriage or change your mom's behavior. Honestly, at this point you need to stay out of it. Don't lie for your mom, but also stop checking up on her and trying to catch her in lies.

Not to condone your mom's behavior in any way, but being a "good" wife and mom can be fulfilling and wonderful but can also be draining. Sometimes when the kids are grown or almost grown the women realize that the life they have lived to that point is over and they want something more. Maybe it's a new career or independence. Obviously the way she is handling her mid life crisis is not appropriate. But hopefully she figures out quickly that she needs to end the affair or end the marriage.

Is it an option for your to move out and start your own life?