I think I have flat head syndrome. I know that babies can have this and it could be treated but I'm 18. Any suggestion? (photos)

Based on your photo it seems that your skull is cone-shaped, with a extra length at the vertex (top of the head). The proper correction of this bony deformity will require cranioplasty, or reshaping of the skull, which is usually done by neurosurgeons and is an extensive surgical procedure. Consider getting a consultation with a neurosurgeon to see if you are a candidate and to learn what is involved with this surgery. Not sure that fillers into the forehead will help. First of all, you will need quite a bit of a filler, and it may make your skull appear even bigger, which is not the goal.