In this paper we report on a case study of a university student (third year of Mathematics course). She was engaged in proving a statement of elementary number theory. We asked her to write the thoughts that accompanied her solving process. She was collaborative and her protocol is suitable to study the interrelation between affect and cognition. We seize in her performance a poor control of the solving process intertwined with emotions and beliefs on the mathematical activity. [For the complete proceedings, see ED489538.]