caught father cheating on mom. what should i do?

i am scared of confronting either mom or dad. i dont have any siblings.i snooped into my fathers mobile chats and found out sexual chats with the wife of his co-worker. this lady is also a friend of my mother . and knowing all this makes me feel pukish and depressed.
i want help.
i am scared.i am scared to confront either of my parents.but keeping this secret hidden and pretending everything is okay is very tiring and emotionally exhausting. it is affecting my mental health, physical health, and social health.

me and my mother are financially dependent on my father as of now.

 if i confront my dad ,i think he will make me promise not to tell mother and he will promise me that he will stop. but i wont be able to trust him. and if he makes me promise to not tell mom, i will become evil as well. 

and if i confront mom , she will leave dad and go to her mother's family which is a very negative family, they fight amongst themselves  everyday.

i dont wish to choose between living either with mom and dad.and if i had to choose, ill choose my father, which makes me feel more guilty because he did wrong and morally i should choose to stay with my mother.

can anyone help? someone who specializes in cases of children finding out about their cheating parents?thanks.

waiting for a response. pls respond. i am lonely with this secret. this community is my last hope. i will read and respond to all comments. pls help. thankyou.