So the other day my mum used my phone to go on Facebook messenger because her computer crashed and I was using mine. When she gave it back it was still on hers and it was on a mans messages. So I scrolled up and read some of them. I was absolutely disgusted. They were more like sex messages. I really don't know what to do. My stepdad is the only dad I have. My real dad doesn't want me and ignores me whenever I see him. My stepdad makes me feel wanted and makes me feel like I have a dad who cares. He's given us everything and has always been there for me and my mum. I know what I did was wrong and that I had no right to read her personal messages, and trust me, I wish I didn't. I really don't know what to do. Should I confront her? It just hurts so bad and I can't handle it. I can't even look at him without wanting to cry and looking at her makes me even more angry. What do I do? :/

Talk to your mom about it, tell her what happened, and what you saw, and tell her how you feel about it.

Your mom should not be putting you in the middle like this. She's not a very good mom in my opinion.