Going back to being friends

Me and my now ex boyfriend were best friends before dating, we always said that no matter what we wanted to make sure we remained friends. I don’t agree with his reasoning for breaking up and we haven’t spoken since it happened except for a very brief interaction at work where I ended up writing on a sticky note asking him to leave me alone because being around him is still hard for me. Last night I got very upset because I’m still not over him but I got worried that we’ll won’t get past this and that I’m going to lose my best friend. I’m also worried that if we stay friends our feelings won’t ever really go away. Does anyone have any experience being friends with an ex?