The old marriage or partnership is over the moment betrayal occurs. If you stay, you are not 'saving the old relationship' — you are choosing to negotiate an entirely new one with a partner whose reliability must be proven from scratch.
If you leave, you are not 'failing.' You are exercising self-respect, honoring reality, and creating space for authentic peace and future joy.
Evaluate progress by observing consistent behavior over months, not emotional promises during crisis.