On again off again will do nothing but teach her you are a pushover and her behaviour will spiral downwards to deplorable. She will seek out other guys because she has it in the back of her head you will always take her back. This is the worst position to be in because the woman will believe she has carte blanche to treat you poorly, abuse you, disrespect you, even cheat on you.
Make it known you will give a girl 1 i repeat one chance per lifetime. If she blows that chance she is out for good. No Time apart, no breaks No second chances nothing.
She ends it. Done. Next. No coming back. You end it. Done. No second chances.
She tries to come back after she pulls" i no longer love you i don't know what i want, i think we should see other people blah blah blah blah." Then pulls the " i made a huge mistake i want you back take me back," The answer is always the same. Sorry. No. It is over. You ended it. Goodbye.
You move forward. Next. She is history.
When a girl has it in her head you will always be her safety net, she will take risks. She will flirt with other guys she will entertain their advances she will cheat, she will treat you badly because she believes you will always be there.
When a girl knows you will throw her to the curb with the rest of the trash and not ever give her another chance, how much bad behaviour do you think she will pull then?