What can I do if my supervisor is creating a hostile environment?

My position was taken away and given to a younger less experienced person, unjustly. I hadn't done anything wrong, but was told the company was "going in a new direction". This person has now made me feel like I am a rookie and don't know what I’m doing. So that has made me feel bad enough about myself, but now things like telling customers that I am "Shooting up" and can't talk to them (while I am really injecting insulin as I am diabetic). I have other severely limiting disabilities and don't have any other options right now than to stay at this job for the paycheck and insurance. What can I do to protect myself from further defamation or slander, what do I need to do if this continues or gets worse, and can I be retaliated on if I do choose to pursue anything? Texas is a right to work state and it just seems like they can tell me to go and say it has nothing to do with age, disability, or anything I might pursue relating to what this person is or has done.