My Evil Stepmother...

So, my Dad got remarried WAAAAAY back when I was probably about 8 years old. He married a woman named Pamela. She is an interesting lady. She can be your best friend or your worst enemy, depending on whether you have crossed her lately. She is a very selfish, bitter and angry woman.
Around the time that I turned 18, I decided to try and reform the relationship I had with her and my Dad. I hadn't found much time to visit them (or, naturally, my half sister and brother). I really wanted to change that. So, I made a concious effort to visit more often, call when I could, and try to better our relationship in general.
It has been about 3 years since then and my Dad has been gone for 2 of them for deployment. I admitt that I should have found more time to visit while he has been gone. But honestly, it was harder for me to deal with his absence while I was there rather than at home. Recently, I was attempting to make plans to visit over Labor Day weekend. I had lost my job, so I didn't have to worry about taking work off. All I had to get in my way is money. Well, 2 weeks before I was supposed to go visit, I got a new job. I also didn't make the cut-off to request Labor Day weekend off.
I called Pam to tell her the news. She wouldn't let me explain why I can't go... she just said,
"Whatever, Elizabeth, I didn't expect you to actually come anyway." Obviously, she said a few other colorfull things, but I was frozed with shock. I was dumbfounded. I didn't know what to say!! I was completely blown away. I wanted to ask if the following weekend would work... but she didn't give me the opportunity. I got off the phone... after she hung up on me... and just sat there for a few minutes.
Then I realized, most of why I was so affected by it what was said, was because of past bullshit with her. I am not over some of the things she did to me like I thought I was. I thought it would be easy to just let go. I spoke too soon. I don't really know where to go from here. I wrote her a letter (telling her EXACTLY how I feel) but I don't feel I could EVER send it to her. I fear that if I cause problems between her and I, that it will cause problems between my Dad and I and the kids and I.
She is truely the evil stepmother from Cinderella.