Is there any way to fix an 8cm long forehead?

There are surgical methods to reduce the size of your forehead.You should first make an appointment with a doctor for an exam to make sure a forehead reduction is possible in your particular case.In general, forehead reduction surgery involves an incision along your hair line and pulling your scalp forward and cutting out any overlapping skin then sewing back the skin. This obviously requires an exam to make sure your scalp is loose enough and you do not have other un-diagnosed cause of hair loss.**Limitation of a foreahead reduction is that sometimes you will see a scar on the hair line (no matter which technique or doctor you choose). Another limitation is that you will not get the rounding on the side corners (oval shaped hair line which many women have).Other surgical option is hair transplant surgery. Hair transplant surgery involves harvesting hair from the back of your head and relocating to the front (one by one) to create a new lower hair line. This also requires an exam.**Limitation of a hair transplant surgery is that you will have to wait for the hairs to grow and it will not be as full as a hair line lowering surgery (no matter which technique or the doctor you choose). Some patients decide on BOTH surgeries to take advantage of what each surgery can provide.

Hairline lowering for the long the forehead. I have done this operation for more than 35 years hundreds of times. The hairline can be lowered with an Irregular Trichophytic Incision so you do not see the scar. You will need your parents permission for the surgery before you are 18.

You did not state if you are male or female, and that is important.   Forehead reduction surgery comes in two types, the first is a surgical procedure that gives you same day results, the second is a hair transplant that takes 6-7 months to see. For men, my main concern for men who have a hairline advancement surgical procedure is in the potential balding that may occur as most balding can start at any age and advance until the mid-fifties. If that happened to you (your risk regardless of your sex) then you will have to get a hair transplant to cover it as a scar from a hairline lowering surgery will show with a receding hairline as the frontal area is the place that genetic balding starts in most men.
For young men, this chance of this happening is fully 99% over time so a surgical hairline lowering is not the procedure of choice in men for a receded hairline and many times, unless you are over 40, you will not really know. The scars that appear, are permanent and you need a hair transplant to cover it, burying it in the new hair placed by the hair transplant. With regard to hair loss following such surgeries, in women it is not common, but it does happen, in young men under 35, it is very common and only the drug finasteride will prevent this from happening but it will most likely not reverse it.