A more permanent solution for jaw slimming than Botox on the masseter? I am 22 years old and have got this done since age 19.

The long term effect of botox can be a semi permanent reduction in the size of the target muscle. In your case, this may be a desired side effect. If you want to pursue a more permanent result, you can consider either shaving down the muscle itself, or a bit of the jawline bone where the muscle inserts. This is done from inside the mouth in most cases.

South Korea's most famous jaw contouring surgeries - v line or square jaw surgery could be a solution for a permanent slimmer jaw line. 
- V line surgery 
The surgery reduces the angular jaw and the width of the front chin to form a slimmer facial lines. 
It's recommended If you have a large angle on the side jaw and a round front chin. 
- Square jaw surgery
The surgery reduces the angular square jaw, excessive fat and muscles creating a slender and defined jaw line. 
A surgeon can recommend the best solution according to your skull X-ray, nerve line and the amount of facial muscles and fat.
Best wishes,
Park Sang Hoon, ID Hospital, South Korea 

A jaw reduction surgery is a permanent option that will give you the slimming effect you gain from botox to the masseter. However do bear in mind that botox is safe to use long term in your masseter, so do have a good think before opting for invasive surgery. Should you wish to proceed, book a consultation with an experienced surgeon who will be able to discuss aims and realistic outcomes with you. Best of luck!
GB

I specialize in jaw jaw surgery and your question is very common in my practice. Botox is safe long term so if it's not a hassle to keep doing it then you can just keep doing Botox. However, we can do a jaw reduction procedure to slim the jaw and masseters permanently. It can be subtle or drastic depending on what you want/like. Consider a consultation to discuss your goals and how to best achieve them. Best wishes.

It is not uncommon to alter the shape of the lower jaw for 'slimming' the facial contours. The change can be achieved by changing  the 'width', or contour/angle, or form of the back of the lower jaw. Also, altering the 'middle' of the lower jaw and chin for specific cases can alter the facial shape. Any of these can be pursued based upon your goal.