Answer
Lanskoy stopped being Catherine’s favourite in 1784. This was due to a number of factors, including his deteriorating health and Catherine’s increasing interest in other men. In particular, Catherine became increasingly close to her aide-de-camp, Alexander Dmitriev-Mamonov, and it was widely rumoured that they were having an affair. As a result, Lanskoy became increasingly isolated and depressed, and he died in 1784.