I can relate. It's so easy to get caught up in catastrophic thoughts like these.
You say that there are "so many things" holding you back, however I would argue that there's only one thing: the fear of uncertainty. That's the only thing you have to address.
The answer to all of these "what if?" questions is...you would just deal with it. Often we get caught up in the assumption that we wouldn't be able to handle some terrible situation, and we forget just how capable we are. Humans are pretty adaptable. Even if some huge, life-changing tragedy occurs, it might be jarring for a while and it might take some time, but eventually you find a way to continue living life.
Right now you're getting lost in thoughts about the future, which is full of unknowns. You're anxious about getting this job, so you're ruminating about these potential outcomes in an attempt to gain certainty and get rid of that anxiety. Instead, what you need to do is learn to sit with that anxiety, and continue to live your life *despite* it. You need to prove to yourself, through *action*, not thought, that you can actually handle the uncomfortable feeling of uncertainty. You don't have to do anything about it.
When these catastrophic thoughts pop up, try to respond to them with, "maybe, maybe not." Bravely meet the uncertainty with more uncertainty, even if it feels wrong to do that. The thoughts aren't the problem, the reaction to them is. That "wrong" feeling only means that you're beginning to step outside of your comfort zone.
Start with baby steps. You don't have to think about your entire future as a waitress, all you have to think about right now is applying for the job itself. That's it. If you get the job, all you have to think about is getting through you first day. Then the next day, and so on. Take things one day, hour, minute, second, and moment at a time.
