How do you know if it's really the last time you break up?

For many years, I’ve had an on again off again relationship. Almost every time we break up (sometimes it’s her, sometimes it’s me) we declare it will be for the last time. Alternately, when we get back together, we declare it will be forever. We have broken up once again. How do I know that this time it will be forever, and what should I do? Help.