People with Multiple Sclerosis (MS) often experience cognitive difficulties such as memory problems, concentration issues, and reduced processing speed. These symptoms can have a negative impact on daily functioning and overall quality of life. Previous research on cognitive rehabilitation has shown that regular training focused on memory and concentration can have positive effects on cognitive functioning, including processing speed, memory, and executive functions that support daily activities. Moreover, fMRI studies (brain scans that measure brain activity) have revealed changes in brain activation following cognitive rehabilitation. Recently, the idea has emerged that a more personalized approach could improve treatment outcomes. Specifically, researchers have identified a link between personality traits and cognitive functioning. Since every individual is different, current cognitive rehabilitation programs often fail to take these personal differences into account. In this project, the investigators aim to enhance the effectiveness of cognitive rehabilitation by focusing more closely on individual characteristics through an app-based training program. Participants will complete a 12-week app training prior to a 6-week cognitive rehabilitation program. The first app focuses on mindset training, supported by a coach. Afterwards, all participants will use a second app designed to train processing speed and memory. In addition to cognitive functioning, the investigators will also examine psychological, (neuro)biological, and social changes using questionnaires and fMRI. This research may provide valuable insights into how cognitive functioning and quality of life in people with MS can be improved. This study is funded by the National MS Fund and is a collaboration between several institutions: the Department of Health, Medical and Neuropsychology at Leiden University (The Netherlands), the University at Buffalo (USA), and Reha Rheinfelden (Switzerland).