I'm tired of dealing with my ex, so I'm not going with her tomorrow to help sell her art.
Why don't you want to go with her to help sell her art? Isn't that rather extreme of you?
She has done nothing but taken advantage of my friendship
I can understand why you wouldn't want to help them if they have taken advantage of you in the past. You can tell them no if you don't want to help them.
I'm just really tired of her taking advantage of me.
And that is okay, anybody would be upset about people taking advantage of them. Especially if your exes. You don't have to be responsible for them anymore.