How do you keep a positive attitude when working closely with someone who is constantly bringing you down?

I’m a grad student who sees clients. So far, all of my supervisors have been really great to work with – I feel comfortable asking them questions, they balance praise with constructive criticism, and I feel like I really benefited from working with them. Now I’m stuck with someone who is a pretty negative personality, makes me feel like a child, etc. I can’t switch supervisors or anything. I’ve always been able to handle working with different personalities but I actually dread our days (a couple days a week) working together. It makes me anxious just thinking about it. Obviously I’m not going to change her. How have you handled a similar situation without losing your mind or obsessing about it all the time?