Are there treatments for naturally thin hairlines?

There is no medical or surgical treatments for "naturally" thin hair. It is who you are.  It is a genetic trait.  If you are considering forehead reduction surgery, insurance does not pay for it.  There are no natural remedies for a high forehead.

Thinning Hair -- Rogaine, Propecia, Viviscal, Progesterone, PRPIf you have thinning hair you really need an evaluation to see if it is genetic or from another cause like stress or autoimmune. If it is determined to be genetic there are many options and traditional treatments like rogaine foam and propecia pills can be used. I have great success in my practice using injectable progesterone and prp to the scalp. Postmenopausal women also benefit from spironolactone. I suggest seeing a dermatologist with expertise in hair loss. Best, Dr. Emer.

Your situation--especially in view of your age--is one that needs to be evaluated in person by a dermatologist. Since what you describe is a cosmetic issue, albeit a very distressing one for you, it is a virtual certainty that any treatment beyond an initial evaluation wouuld not be covered by insurance.

What you are describing is difficult for me to imagine so I would suggest that you see a good doctor first, and then after the doctor examines you, you might have an answer. Without an examination, I can't give you a recommendation.