English is very short on "rules" - we don't have a "Royal Academy of the English Language" like some languages do - but the forehead here sounds weird to US English speakers. My means "belonging to me"; the is ambiguous. Since your body parts are very unambiguously attached to you, the is a strange choice at best.
I will wash my hands: perfectly natural and fluent.
I will wash the carrots: perfectly natural and fluent. Even if the carrots belong to me, the is fine if we are referring to some already-known carrots.
I will wash my carrots: natural, but does put an emphasis on my that implies there might be some other people's carrots too, which I am not going to wash.
I will wash the hands: very strange-sounding, unless you are referring to some disembodied hands! It's OK if you specify who they belong to, like "I will wash the hands of the children", but this kind of construction is almost never used for things that belong to you.