How to get over my childhood past?

I was in grade 5-6. My mom took me on a holiday for a few day without my dad. I was not prepared. She picked up someone and introduced to me and my sister. That’s her boyfriend that she has cheated on my dad. I wouldn’t take it this hard, if my parents divorce. But they’re still together. I have to cope and help her hide her affair. Also my dad he’s a cheater too, with many of his one night stands. It’s been more than 15 years until now, this life of mine still continues like this. It’s hard to cope, it’s hard to pretend, hard to help them lie, hard to witnessing everything. I remember my emotional that day when she introduced her affair to me, I just pretend to be happy for her happiness. Until now I still pretend.
How to get over my childhood past?
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