I'm thinking of declining to be a bridesmaid for one girl in my group of friends
Why do you want to decline the duty?
I think she's a bit high maintenance for me. When things don't go her way she can be so difficult and I see this making the wedding planning stressful. I don't think I could help her with this.
They would not have asked you if it was not important to them. I think that you should agree to it because it is important to support your friends.
This isn't fair, it's so annoying. I need to stop them from taking advantage of me.
What makes you think that they are taking advantage of you? In most cases, friends are usually wanting to have you in their wedding because they care about you