A new semiclassical decoupling procedure for rotational projection states in rovibrationally inelastic atom-diatom and diatom-diatom collisions is developed. Computed vibrational self-relaxation rate constants for para-H-2 and ortho-H-2 are in good quantitative agreement (within a factor of 1.5, except for the lowest temperatures) with experimental data over the investigated temperature range 50-2000 K. This allows us to hope that also more detailed (nonmeasured) rate constants for rovibrational state-to-state transitions in molecular hydrogen, calculated by our new model, are sufficiently accurate for astrophysical applications.