Private car-based transport poses a global concern, including greenhouse gas emissions, traffic congestion, and associated health risks. These challenges necessitate a transition to more sustainable mobility solutions, such as carsharing. Shared cars offer potential benefits by reducing private car usage, improving vehicle utilization efficiency, and lowering transport emissions. However, the success of this transition relies on individuals voluntarily altering their travel behavior. This study uses the stage model of self-regulated behavioral change as a framework to investigate the factors driving behavioral shifts from private cars to carsharing. Through six focus groups conducted with 30 participants in London, UK, and subsequent content analysis, the research identifies the behavior process and key determinants influencing individuals' intentions. The study also examines the potential of shared autonomous vehicles as an extension of carsharing, exploring their role in shaping future mobility systems. By offering a nuanced understanding of the nature of behavioral change and highlighting factors, this research provides insights for designing policies and carsharing services that address barriers and leverage facilitators to promote sustainable mobility.