Should I stay friends with ex?

Hello,
month ago me and my girlfriend broke up. We've been together for almost 4 years. But we ended it friendly. And we decided that we want to stay friends. First week everything was good. But then I found out that she have a boyfriend. After that I felt depressed. But we met and talked about this, because she is the only person I cant talk to about everything. But almost everyone think that it's not a good idea to stay friends with ex. Is it really bad thing? I don't know, I want to stay friends with her, because 4 years is not a short period and loose a person because of break up is silly. What's your opinion about this?

yeah the people are right, you should not friends with her, because you are not ready of losing her.
see,, you feel depressed when you find out that she have boyfriend. being friends with her will make you more depressed.
you should find a new one, you should start a new relationship.