Past Perfect Continuous or Past Continuous

Past perfect continuous should be used to show something started in the past and ended in the past. Go with that.

Is my sentence with Past Perfect Continuous correct? Are you a native English speaker? Thanks. Waiting for your reply!

It's not really a matter of correct, Duhashostka. The issue is whether the context warrants using a certain structure

Why not simply use the simple past?

"This woman kept to a diet from May till July last year" .

Usually there is some reason that we choose the PPC or the PC.

"This woman had been keeping to a diet from May till July last year when she unexplicably fell into a coma.

"This woman was keeping to a [specific] diet from May till July last year until she developed serious health problems"