Most of my facial features droop downwards and it makes me look upset/sad. Fix for downward smile? (photo)

Lip augmentations with fillers would be a great option to improve the fullness, suppleness and symmetry of the lips and provide improvement to a downward smile. I prefer to use a micro cannula to inject the lips to prevent bruising. 
I hope this helps.

taylrc.   It appears from your photos that filler, like Juvederm Ultra plus would be just the antidote to your frown.  Correctly placed, it will lift the corners of your mouth nicely.  Fat transfer is a more permanent option.  Be sure to consult with a board certified plastic surgeon. Good luck!

Many thanks for your question and the photographs. 
A combination of Botox and dermal filler treatment would be an excellent option for you to consider. 
There is muscle, called the depressor anguli oris, which may be overactive and is causing your mouth to turn downwards. This muscle would need to be accurately injected with 2-4 units of Botox per side. Injection into the wrong muscles would result in an asymmetric smile. 
Dermal filler treatment placed at the corners of the mouth and just below the mouth - in the marionette lines - will also help to lift the corners of your mouth. 
Please see a certified medical practitioner with plenty of experience with plenty of experience with these types of treatment. Good luck.