My mom has a secret boyfriend

My mom and my dad got divorced about 3 years ago. I’ve never really liked my mom because she just doesn’t treat my brother and I like human beings. Whenever I’m with her my mental health gets so much worse. I go to my dads house every other weekend and always look forward to seeing him and getting a break from my mom. She is so much to deal with and sometimes I just can’t take anymore. Shortly after my parents got divorced and before I had a phone I went on my moms iPad to text a friend. I came across some messages between her and a guy talking about how they want to be together. I’m not sure if the messages were sent before or after my parents got divorced. I started seeing my mom FaceTime sometime everyday for hours at a time. She told my brother and I he was a friend she used to work with but we had already connected the dots. I constantly saw messages between them talking about how they love each other. I didn’t mind much that my mom had a boyfriend but the fact that she was keeping it a secret made me mad. I confronted her about it one time but she just denied everything. Since then I have seen messages between them talking about a ring and getting married. She went to Idaho for Aaron’s sisters wedding. That’s her boyfriends name Aaron. She came back with a ring on her finger and I asked her about it one time but all she said was we’ll talk about it later.she only says that when she’s not gonna tell me. I have nothing against Aaron and actually quite like him. My. I’m wasn’t lying about him being an old coworker. He was and they used to be pretty good friends. His daughters who are grown now used to babysit my brother and I when we were little. I wish my mom would stop hiding it from me and just tell the truth. She can’t keep it a secret forever especially if they’re married or engaged. My dad doesn’t know about him either and I don’t know how he’ll react when he does find out. I’m glad my parents got divorced because i think my dad is too good for my mom but i know he loved her. I’m not sure if she and Aaron were together before my parents got divorced or not. I really hope they weren’t because if I find out my mom cheated on my dad I don’t think I will ever be able to forgive her. Aaron lives in Virginia and my mom is there right now for a supposed work trip. What should I do?