Optical properties of transition metal dichalcogenides monolayers are controlled by Wannier-Mott excitons forming a series of 1s,2s,2p,... hydrogen-like states. We develop the theory of the excited excitonic states energy spectrum fine structure. We predict that p- and s-shell excitons are mixed due to the specific D3h point symmetry of the transition metal dichalcogenide monolayers. Hence, both s- and p-shell excitons are active in both single- and two-photon processes, providing an efficient mechanism of second harmonic generation. The corresponding contribution to the nonlinear susceptibility is calculated.