Recent observations of molecular clouds indicate that dense filaments are the locations of present-day star formation. Therefore, it is critical to comprehend filament formation and evolution since these filaments provide the initial condition for star formation. The filament width is a significant factor since it determines the fragmentation scale through self-gravity. Observations suggest that the width takes a universal value of 0.1 pc. However, theoretically, the width of supercritical filaments should shrink due to self-gravity. Recent studies suggest that massive filaments (~100 solar masses per pc) are bound by slow shocks resulting from accretion flows onto the filaments. As the slow shock wavefront is known to be unstable (slow shock instability: SSI), the accretion ram pressure can convert into thermal/turbulent pressure at the shock front, maintaining the width. Ambipolar diffusion (AD) is effective in dense filaments. Using two-dimensional MHD simulations, we investigate filament evolution via SSI considering AD and discover a new mechanism (named "bullet mechanism") that drives turbulence. We also conducted a three-dimensional MHD simulation with self-gravity. We found that the bullet mechanism can sustain realistic filament width even for a filament as massive as ~ 100 solar mass per parsec.