Not to be cliche, but you can only truly be friends with an EX when you know longer want to be friends with them. Its a catch-22.
Thats why you'll hear stories from people like they stopped talking/seeing their EX for like a year or multiple years and accidentally ran into each other and slowly developed a new friendship . . . which usually doesn't invovle being "best friends" i.e. more or less constant contact . . if you talk to an ex several times a week . . . well that doesn't seem really heavy on the EX part.
When you keep talking early into the break up, someone is always forcing it, maybe both. Its understandable, there is definite emotional dependancy to any relationship and the strain is much like withdrawal, it hurts. It can hurt so much its like being sick. And people will do almost anything to avoid being sick. And every time you talk to an EX it alleviates that illness a little . . . its still bad but often not as bad.
But, if you establish a relationship right away and end up forcing something . . it creates a weird kind of cathexis that sustains the old relationship in some pale imitation of itself, and make it really hard to have a new and good relationship with someone new and worthy.
If we were writing it as a screenplay, you would establish a friendship with an EX that would influence all your future relationships negatively, all your friends would see it, but you would deny it. "Oh we are just friends."
Until finally you met someone really worthy and your relationship with your EX ruined it . . . and you would finally realize what was happening but it would be too late for your new relationship and your old one . . well you an never go back. Leaving you older, wiser and alone
Bad movie.
The variations are even worse. The new guy forgives you and takes you anyway. What a loser because you would have already ruined it. Or you go back to your ex . . another loser deal.
Sigh. Note: "You" in the above is not meant for anyone particular in this thread.