There is no easy answer for this question. There are three ways to change the shape of a lip: one is non-surgical, and that is by using a filler of some kind. The second is the placement of a lip implant, and that is surgical. The last is changing the shape of the bony skeleton, which can alter how much teeth you are showing at rest and when smiling, but does not really alter the lip. The third option is covered by insurance in some cases and is done by craniofacial plastic surgeons. The first two options are truly cosmetic and not covered by any insurance.
If you are not a candidate for craniofacial surgery and you cannot afford a filler, then I think you need to accept the shape of your lip as it is. Good luck.