Personally never been in many bad relationship long enough for me to feel that. I've had very mutual attraction where we both supported each other for the most part. If it gets to that early then that's a sign it's not working. If it feels that bad it can't be beneficial, then that's probably a cut off point (depending on length, after years a discussion is in order anyways).
The bad one? Perhaps had blinders on and didn't see but looking back didn't see as much take to the give. It's easier to see looking back but don't take that one bad issue is a reason things can't work either. Sometimes a good talk (the horror of any tv show) can work things out so everyone is at the same expectations. If not then .... That happens too, more often than finding a long term partner. Otherwise there'd be hardly any dating.
In the end communication about expectations are the key, if they don't line up you probably won't as a couple either.