Exciton-polaritons (EPs) are valuable for investigating macroscopic quantum coherent phenomena. However, overcoming the rapid decoherence of EPs under ambient conditions is challenging, even using delicately fabricated optical cavities. Here, we show pronounced coherent dynamics in thin flakes of WSe2 supporting self-hybridized EPs. Ultrafast two-dimensional electronic spectroscopy (2DES) resolves two types of quantum beating: Rabi oscillation with a period of ~ 24 fs and a slower, ~ 50 fs period oscillation, revealing coherent coupling not only between excitons and cavity, but also between polaritons and dark states. Rapid evolution of 2DES line shape indicates a change of dominant optical nonlinearities from Rabi contraction of polaritons to many-body interactions of excitons in the first 100 fs, marking the transition from coherent to incoherent polariton excitations. The results are rationalized by a three-coupled oscillator model considering the inhomogeneous cavity field distribution. We also demonstrate coherent quantum beating between A exciton and its excited Rydberg state A* under ambient conditions. Our results shed light on the coherent control in readily accessible semiconductor thin flakes.