Lenni, don’t beat yourself up over what wasn’t really flirting and certainly wasn’t cheating on whom wasn’t really your boyfriend.
I don’t suggesting lying. In fact, I don’t reply, “all is well,” to the standard, polite, “how are you?” if that would produce direct falsehood. On the other hand, you can deflect the question, say that you’re coping or that you have some problems but nothing major or nothing that would hold you down.
And yeah, it must be an awkward situation having an ex-boyfriend for a friend, especially with stuff like emotionally caring, needing daily contact etc. However, while you can have a full friendship, you can’t have a half-relationship. Perhaps some grace period may be in order, so that, for a while, you still give him more time than you would give to a friend, and tolerate his expressions of protectiveness, care, even attachment, when they don’t get out of line, but at some point you need to start treating it like a normal friendship. Better sooner than later. Unless you want to come back to the relationship, but this shouldn’t be done half-heartedly (should be a weighted and weighty decision).
