Managing relationship conflict is a difficult yet crucial task for romantic partners. We investigated how imagining a conflict’s resolution affects the engagement in resolving that conflict. For instance, people may indulge in positive fantasies about their future, or they may mentally contrast their desired future (successful conflict resolution) with the main inner obstacle (e.g., anger, frustration) in the current reality. In two experiments, we tested whether mental contrasting increases engagement in resolving ongoing relationship conflicts participants perceived as important (i.e., high incentive value) and solvable (i.e., high expectations of success). In Experiment 1 (N = 274, from predominantly satisfied relationships), mental contrasting compared to indulging improved the resolution of solvable conflicts over two weeks. This effect did not extend to conflicts of highest importance, which we found to be particularly severe. In Experiment 2 (N = 270, from less satisfied relationships), mental contrasting, compared to indulging and to working on a concentration task, increased mental engagement with highly important, more severe conflicts over two weeks. Results suggest that mental contrasting helps in resolving solvable conflicts. When facing more severe, less solvable conflicts, however, mental contrasting stimulated mental engagement with conflicts. Our findings suggest that teaching people mental contrasting may complement existing conflict management approaches (e.g., in counseling or in self-help apps).