How can I fix my forehead indent? I've always had it. (Photo)

You have very interesting anatomy.  In most people there is some cross over of the frontalis muscle in the midline of the forehead but in you the muscles in the midline are absent causing a decrease in volume and the indentation.  Soft tissue dermal fillers will address this problem nicely but not permanently.  

What you have is a normal anatomical variance. What you see is the two heads of the frontalis muscle. The best approach is to consider injectable fillers or fat transfer. We now have a few fillers on the market that are longer lasting. Voluma and Bellafill are two fillers that you might want to consider. Both have pros and cons that your board certified surgeon will review with you before proceeding. 

The best way to correct a contour depression of the forehead is with fillers. Fat can be used as well. Cranioplasty requires incisions and I would not recommend this for a midline contour problem