I recommend this state of mind if you can embrace it. I first came across it as "Today is the day I will die." While dying is significantly more impactful than being fired, it also frees you from thinking too hard about how this thing you are doing might kill you. It is sort of an extreme of course.

I've found the best employees often have let go of the fear of being fired. They can speak truth to power, stand up for what is right as opposed to what is expected, and they generally leave the place better than they found it. It was interesting watching people at Sun change when Sun going public suddenly meant those people didn't have to work if they didn't want to. Much more engaged, much more willing to look at alternatives and to take risks in pursuit of a better outcome overall.

That said, the philosophy only works when you also have a high degree of integrity. If believing that you're going to get fired tomorrow means you're totally ok with emptying the coffee tin people put money in to cover the cost of the office coffee service, then that doesn't work so well :-).

Sometimes I'll do the exercise where I write down in my notebook all of the things I can think of to worry about and then rank them from most important to least. I find it can help with perspective.