Grid-forming voltage-source converters (VSCs) tend to suffer from sideband oscillations in stiff grids. Conventional state-space eigenvalue analysis or impedance-based stability analysis based on generalized Nyquist stability criterion can only analyze how specific control parameters affect the overall system stability, which ends up with less insight into a controller design-oriented analysis. To tackle this challenge, this article starts with the impedance modeling and stability analysis of grid-forming VSCs in stiff grids. Then, an impedance decomposition approach to characterizing the impacts of multiple control loops is proposed, which benefits in a controller design-oriented analysis for stability enhancement. Finally, simulation and experimental results are provided to validate the approach.