I have bump/dent on my forehead. I'm wondering what caused this and what can I do to fix it? (Photo)

One of the most important parts of diagnostic capabilities is the ability to perform a physcial exam, to see if the bump you have is fluctuant, soft, hard, scarred, etc...  Without being able to that and by appearance and history you provided, most likey scenario was that when you had your trauma and the ensuing bump, the area formed a seroma that then fibrosed forming a scar that now manifests itself in what you see.  There are many ways to appraoch this and improve the appearnace of it.  Best would be to seek a cosmetic surgeon or a plastics surgeon to get their thought on options.  Also look into non invasive therapies and explore all your options but with the technology avaialble these days in medicine and cosmetics, you will no doubt be able to have the appearnce you please.

These bumps are bony and the only way to address this is to have it shaved down by an expert, usually a maxillofacial surgeon