GF cheated

Hello guys,

Just want an advice from you all.

I'm 28 male, I'm in a relationship from past 1.5 years, my gf has cheated on me more than twice. I noticed she sometimes talk to some people at nights, and last week, she was taking a bath, and someone called on her phone, the number was saved as a normal name (not like baby etc) , i picked the call, and i said hello the person hanged the call.

I asked my GF about it, she aaid it's just her friend, but she didn't let me meet that friend.

I know she's cheating. I like her from past 4-5 years. But i don't know what to do, any advice?