(1) Background: Work adaptation can be seen as a skill to be acquired, allowing employees to acquire new practices, integrate them, and transform themselves until they reach the desired goal. Previous studies show that this is influenced by organizational resources such as optimism, resilience, and proactivity, as well as work motivation, to allow employees to constantly adapt to their environment in order to achieve a better quality of life at work. The purpose of this study is to determine the extent to which work motivation plays a mediating role between personal resources (optimism, resilience, and proactivity) and work adaptation. (2) Methods: The sample consists of 273 teachers who responded, on a voluntary basis, to paper questionnaires distributed to schools. Structural equation modeling (SEM) is used to test the above assumptions. (3) Results: The results indicate that self-determined motivation plays a full mediating role in the relationship between the three personal resources and work adaptation. (4) Conclusions: The main conclusions demonstrate the importance of these three personal resources, optimism, resilience, and proactivity, in maintaining work motivation and work adaptation. In addition, in order to promote a positive influence of organizational resources on work adaptation, it is essential to activate motivational factors in the workplace.