“Common sense tells you that if somebody will lie and cheat with you, they will lie and cheat on you,” Dr. Phil says.

That’s why it’s important to be honest with yourself about the situations you are in.

“Don’t lie to yourself, even if it’s what you want to be true,” he continues. “You cannot be a winner in this world if you lie to yourself.”

On Friday's episode, Dr. Phil speaks with an 18-year-old who is dating a married father of two but says she believes her boyfriend will eventually leave his wife – and to the man’s wife, who says she’s going to fight for her marriage.

Watch more from this episode here.