We performed a quantum Monte Carlo study of the exciton-polaron model which features the self-trapping phenomenon when the coupling strength and/or particle momentum is varied. Accurate data for energy, effective mass, the structure of the polaronic cloud, dispersion law, and spectral function became available throughout the crossover region. We observed that self-trapping cannot be reduced to hybridization of two states with different lattice deformation, and that at least three states are involved in the crossover from light- to heavy-mass regimes.