My dad is cheating on my mom

Two years ago, I found my dad snooping on hookup websites online. I was completely devastated, and it took months just to feel like myself again, let alone look at him the same. However, over time I learned to cope with it, and convince myself that nothing ever happened. But last night, I saw him doing it again. I couldn’t believe that this was happening again. My parents have been married for 20+ years, and he chooses now? I can’t sleep, I can’t eat, I can’t be in the same room as him for more than 5 minutes, especially when my mom is there too. I don’t even know what to do with myself. I can’t tell my mom, she may already know if I’ve caught him twice just as a teenager, so please don’t recommend I do so. Also, bringing it up to my dad is not an option. I know he would only deny it, and my father has made me slightly anxious my whole life anyway, so this would only amplify my fear and anxiety. I just need advice on how to cope with this, because I can’t go through this again like I did two years ago.