Do you have bangs? Whether you do or not it could be fungal acne, which isn’t a fungi at all, it’s from an infection from hair follicles on your face. Does it itch at all? To figure out the solution you’d have to find the root cause. Forehead acne has many possible causes, like pore-clogging hair products and skin irritation from hats or makeup as well as stress, hormonal changes, and certain medications. Pimples usually go away after 3 to 7 days usually. But rosacea, folliculitis, and other skin conditions can also cause them. But since you’ve have them for this long and they haven’t gone away, i would talk with a healthcare professional if they don't, like a derm.