This has been a great shock for you, I'm sending you a big hug. Whatever you do, don't go to your dad with this news. Its hard to believe but your dad may already know about this other man, and he may even be ok with it. Things are obviously not good in their marriage so they may have made an agreement to see other people but stay together for you and any siblings you may have.

If you had not looked at your mums phone you may never have found out, and this 'affair' would have gone away without you knowing and being upset. But you cannot forget what you have seen and heard. Next time you and your mum are alone, explain it all to her. Apologise first for looking at her phone and then tell her everything.

I do hope it all goes well. As you grow older you will realise that sometimes adults make mistakes, or make unusual decisions in order to cope with unhappy marriages.