Well I used to be a pet sitter, caring for many different animals including cats.
I don't know if this is something you are doing, but it's best to have the pet sitter come over and visit while you are there, learn the routine, a show of your house, meet and play with and even FEED the cat, if you could even do this a couple times prior to you leaving that would be great.
If you know the pet sitter personally, you can even ask them to leave an article of their clothing at your house for about a week prior to you leaving (something they have been wearing and unwashed), and let the cat sleep on it or do whatever she wish's. This will help her get accustomed to the cat sitters smell, and see it as a good thing, as a sign of home.
You should also try and get the pet sitter to give the cat one of it's shots while you are still around. Just have the pet sitter do a few or more things you would normally do while you are there. A lot of clients would actually ask me to go out and walk their dogs with them before they left and it worked out great!
One of my cats Asim, has anxiety and depression issues. I know how hard this can be.
Make sure your cats has LOTS of interactive toys and cat purches and cat tree/scratching posts to play with while you are away. Leave the TV on for the kitty if you would like, or a radio, something that's normally on while you are home.
And you should also talk to your cat and explain to them the situation. There has been times when I just up and take off for a day or so and forget to tell Asim, and those are the times he decides to pee on things and otherwise destroy my house. But when I talk to him tell him where I'm going, that I will be back soon and that I love him etc etc, he does just fine!
We are actually leaving June 19th and the 20th, so I'm going to have to find someone to come and take care of all 5 of my animals during that time.