I'm 24 and have a younger sister who is 21. My parents marriage has always seemed well. The other night I got on my moms tablet to look up some travel information. When I opened up the web page it was her email and I looked at what it said. She had a long email to a guy explaining how she had had an affair with a man 12 years ago and was currently seeing someone as well. I was honestly shocked, disgusted, and angry. I never would have guessed my mother would do something like this. Its tearing me up inside knowing that she is being so selfish and heartless and willing to not only tear my family apart, but someone else's as well. It has been a few days since I saw the email and I have cried myself to sleep every night and have no idea what to do. My dad works very hard and is one of the sweetest and most selfless people you could meet and deserves better than having his wife cheat on him. I don't know what to do and need advice. I feel like I can never trust my mother again and feel like no matter what happens, I will never be able to be close to or even want to talk to her again.

It is sad to discover that. But you are an adult and you should understand that your parents are responsible for their own actions.

"I feel like I can never trust my mother again"

I can imagine after your snooping episode she will feel the same way about you.

"My dad works very hard and is one of the sweetest and most selfless people you could meet and deserves better than having his wife cheat on him."

Although you are a member of the family, you don't know and understand the complete dynamics of their relationship. I don't condone cheating under any circumstance. But she is your only mother and before you go ruining that relationship, realize you only get one mother one time. Once she is gone from your life, she is gone. Parents make mistakes and the good thing about making mistakes is our loved ones love us so much they are willing to forgive.

What are your basic choices?
Do nothing.
Confront your mother.
Tell your father.
Tell your sister.
Some combination of the above.
Generally, most of the world will say It's None of Your Business and You Shouldn't Have Read the Emails. There's even the possibility that it's all a total lie for some reason. Or your sweet dad had an affair or affairs, or refuses to so much as hug your mother, much less have a sexual relationship. Or it was all HIS idea for some reason. You must never ever take what you think you see as fact until you know for sure, and it isn't your right to know for sure.
If you do decide to confront her, do it privately and gently.
I personally would not tell anyone but her, and I don't know if I would talk to her or not. I think I might, but you and your sister are adults, and your parents are adults. Maybe your father doesn't want to know! Maybe they arranged it that way!

My mother confided in me about an affair, a million years ago (I was maybe late 20s?). It was awful. I wanted to run to my dear sweet dad but didn't. She tacked on the added burden of 'You are the only person I can tell' routine.

If the anxiety and anger is overwhelming then take a deep breath and talk to your mother as mature person. That's all you can do. Stop talking to her or getting angry will not going to solve the issue.

Joy said it all - and well. Why, when you realized it was not addressed to you, did you keep reading? I agree with Oliver - you'll never trust your mother again and if I were your mother, I'd feel exactly the same about you.

You are 24 and living with your parents, I gather? Or something else?

These words make me wonder just what the family dynamics are: "My dad works very hard and is one of the sweetest and most selfless people you could meet and deserves better than having his wife cheat on him. "

Your dad. His wife?

Start with letting the shock anger and disappointment subside. Let the dust settle on your own emotions before you do anything. Most times a cool head makes the better decisions.