Don't you hate those "it depends" answers? But that's the only one that fits here. As much as you may want to maintain communication or friendly relations with someone you've been romantically involved with, if the relationship ended badly the best thing can be to just let it be and move on. Easy to say, I know. But it all depends, in some cases if the anger which came out of the breakup wasn't too much, or you just made a few regrettable remarks out of being hurt, there's always room to mend some fences and be friends. But you need to ask if that's all you really want, or do you really want to re-kindle the relationship? If you try without a clear understanding of what you're after, you can end up causing both of you even more pain.
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Scientia est Potentia