Amber,
I think anyone with anxiety sometimes fears that they are goind to crack up. What you should do is wait til you are alone and TRY to crack up. I mean really give it a shot, scream, yell, stand on your head even. Once you do this, you will see that you truly can't crack up, no matter how hard you try and you will realize how silly the fear actually is.
As far as irrational fears, I reccomend going to the doctor. My doctor put me on 40 mgs of Celexa and most of my irrational fears are gone. I am now in therapy to kick the rest of them. However, if you don't have the money for that, I highly recommend buying "The Feeling Good Handbook" by Dr.David Burns.
It is a great book on how to change your negative thoughts into positive ones. It cost me $17.95 at Barnes and Noble, and it was well worth it. If you do workbook excercises faithfully, it will really make a difference in how you feel. Good Luck!