I have developed some indentation on my forehead. Why has this happened and how can I fix it? (Photo)

Thank you for your question. I suggest that you move forward and consult with a board certified facial plastic surgeon. You may be a candidate for fillers rather than a surgical procedure. 
Best wishes,

Thank you for your question and for your pictures. To obtain an accurate diagnosis of the indentations on your forehead, you will need to see a board certified plastic surgeon and get an in-person consultation. When you do, talk to them about whether a filler might be an option to correct the issue. It may add enough volume to minimize the indentations and alleviate your concerns. Other options to consider, which might offer a more permanent solution, would be bone grafts. Your plastic surgeon will be able to recommend the best options to get the results you want.

It is hard to say based on these pictures whether the forehead indentation is bone or soft tissue-based. Regardless its long-term solution if probably from a bone-based augmentation approach. But I would first try an injectable filler approach and see if that makes an adequate correction before considering any form of central forehead augmentation.