First, not fulfilling a penance isn’t under “pain of sin”.
Second, priests can’t give penances that last in perpetuity. Penances are finite acts.
Third, there is nothing magical about the rosary. You can avoid sin with our without it. Don’t make avoidance of this sin contingent on some sort of compulsive routine.
Get some spiritual help. It sounds like you need some clarification on the role of penance in confession.