What can I do to fix my negative canthal tilt? (Photo)

You may have a negative or neutral canthal tilt- it is hard to say based on that photo. What you definitely have is scleral show, which is where you can see the whites underneath the iris. To correct this one of two (or possibly a combination) of treatments can be done:
1. A canthoplasty where the outer portion of the lower lid is detached and then anchored to the bone in a higher position to get the lid to sit higher. This may correct the scleral show and create a positive canthal tilt.
2. Fillers to the lower lid. You may have what is known as a negative vector where the cheek is quite flat and therefore cannot support the lower lid that well. Adding filler to the lower lid will provide volume which can then act as a structural support to buttress the lower lid into a higher position, potentially correcting the canthal tilt. Filler or fat grafting to the cheek could also be done if it is indeed flat.

I recommend seeing a plastic surgeon for an in person consult and exam.