I highly recommend that you get in to see a doctor
about this issue, especially since it has been persisting for some time and is getting bigger. You could see either your primary care doctor
or a dermatologist
One possibility is that this is a cyst under the skin. Cysts are very common growths which can occur under the skin basically anywhere on the body. Most of the time cysts are benign, however if the area is getting large and is on the forehead, it is a significant cosmetic problem. Therefore, your doctor might recommend that you have it removed, if this turns out to be a cyst. Removing a cyst is a simple office procedure that can typically be done just with some local numbing medicine.
Another possibility is that this could represent a skin cancer, especially if you have had a lot of sun exposure or tanning in your life. Skin cancers are typically very slow growing and can be completely cured if they are diagnosed and removed. If your doctor is concerned that this might represent a cancer, they will probably recommend a biopsy which, again, is generally a quick and safe procedure that can be done in the doctor's office.