The power self-synchronization control (PSSC) reduces the grid strength by utilizing the filter inductance and grid line inductance to transfer active power, which effectively mitigates the sub-synchronous oscillation issues induced by grid-forming inverters under stiff grids. Nonetheless, the transient instability phenomenon of grid-forming converters is more significant as the grid strength decreases. To address this issue, this paper proposes a transient stability enhancement strategy to make the PSSC-based voltage source converter (PSSC-VSC) adaptive under various operated conditions. By employing the phase portrait method, it is indicated that the PSSC-VSC is more susceptible to instability in weak grids compared to the VSC based on power synchronization control (PSC-VSC). Further, the concepts of virtual power angle and virtual power are introduced, and an orthogonal power-self synchronization control (OPSSC) is proposed to guarantee the existence of the stable equilibrium point (SEP) under different grid voltage sags, thereby ensuring the system's transient stability. Finally, the effectiveness of the proposed strategy is validated by the simulation results.