It is possible that you are still going to be angry. It is possible that they first time you make a foray into being friends you are still going to sound pretty angry. You are free to tell them that you are still pretty angry. They are free to tell you whether or not you being angry is something they can deal with.
You are not free to punch them. Again, I want to be amazingly clear on that.
If you are too angry to stop yourself from saying things that are very angry in every interaction with your ex, that is not a good friendship. I mean, maybe it is if you and your ex both enjoy it, but I guess it’s not my idea of a good friendship. If that is happening, maybe give yourself more time. You can give yourself years if you want. Again, Facebook, man. It’s just not that hard to pick up relationships after a break. We’re all going to live for a very long time. You don’t need to rush yourself into anything.
Look, it is probably never going to be quite a normal friendship, because you’re never going to complain about your significant others in the way you would with someone you hadn’t dated. When asked about your romantic life you are likely, defensively, going to reply “it’s fabulous. We are heading to sea on his yacht for he is the Prince of Spain.” So that will always be a bit weird. At least that happens to me. Maybe there are people that does not happen to, but I find that is one of the weird things about being friends with my exes.
The trade-off for that is that you will have specific memories of stuff he does that are unknown to most people. He will have similar memories of you. Like how you sleep. It’s nice to have someone who knows how you sleep. And they’ll always be able to reassure you that you’re good in bed, so that’s a plus. They’re actually obligated to do that. That is in the “being friends with an ex-boyfriend” contract.
And if you felt rejected by them because you somehow were not desirable enough, well, I guarantee you that if you become friends with them, at some point, in the future, they will stare at you with undisguised lust and longing. You dated. Those feelings are always floating around on some level. Then you’ll get to decide whether or not you sleep with them. I mean, probably don’t. Or do! I can’t tell you how to be the hero of your own story (except that it involves not punching people. I believe that quite firmly).
Don’t become friends as some kind of long-con to make that happen, but that moment will probably happen. That is not the same thing as “winning your ex back.” Again, that is a different story, and one which involves spellcraft.
And, of course, you are not obligated to be friends with anyone you don’t want to be friends with. You may legitimately think they are a bad person and decide you do not want them in your life.
You are free to do that. But then you need to let them go from your life, and let the angry emotions that accompany them go. Otherwise they’re still in your life, just in the most negative, incapacitating way possible. And they can’t be. Because, honestly, those mice in Victorian costume dioramas aren’t going to make themselves.