Was the loss entirely self-inflicted?

Was Britain's loss of sea control caused only by its own mismanagement rather than enemy action?

No. French victories contributed, but poor British command of those battles made the losses partly self-inflicted.

French victories contributed to Britain's wartime loss of sea control. British command of those battles was poor enough to make management failure part of the explanation. The defeats combined enemy success with shortcomings in Britain's own conduct of operations.