How can I fix a downward smile ?

Thank you for your question and I am sorry to hear of your frustrations with your lips and smile.  Unfortunately without an in-person examination or full series of photographs it is difficult to offer definite advice.  I would see a board certified plastic surgeon in order to discuss your concerns and see if there is a treatable cause.  Best wishes. 

A downward smile (drooping mouth corners) is not treated with Botox injection into the masseter muscles. It is treated by Botox injection into the DAO muscles. This treatment is very effective but only lasts for 3-4 months and will require re-injection. Drooping mouth corners can also be treated with thread lift (lasting 12-18 months), with fillers, or a combination. It is not possible to determine the best treatment for you without an evaluation or at least some photographs. Good luck.

Elevating the corners of the mouth can be done with a combination of filler, such as Restylane or Lyft and Dysport/Botox in the DAO muscle. It is hard to comment of the jawline and your smile without pictures.