I've had the im going to mess this up feeling about quite a few things. I understand that feeling. Its not a good one. Its like you try your best and then its not good enough, or you make a mistake, or you doubt yourself in the mist of everything.
It's taken me a long time (and I'm still in the process) to understand that I'm human and therefore will end up making mistakes here and there. I use to beat myself up about them alot. I'm learning to let go of them because crashing down on myself won't fix the mistake (its in the past).
I think about opportunities, friends and relationships that I've wrecked due to drinking (not to mention the relapses that are on my mind).
But I've been beginning to realize that sometimes it takes some harsh beatings and large losses to develop the drive and focus needed to eliminate the common denominator in the problems I've caused myself. Number 1 for me was alcohol. Always more damage done than good times.
Good luck to your new job and go in with a positive attitude for yourself. Let it be a motivator for change.
Prayers for your success,