I don't want to put forth this as a diagnosis of your particular situation, but as someone who's been through similar work situations (being made redundant from what I though of as a reasonably secure position through no fault of my own), I want to ask if you think it could be the speed at which you were summarily fired and then the difficulty finding a new job in today's economy that was a shock to your previous sense of security?

If, like me, possibly now you realize that it can all just be suddenly taken away from you. You might feel on-edge constantly.

Being constantly wary and worried all the time can be quite draining and leave you exhausted. Especially if this new job is better than the old one. Trying to be always on-guard to make sure you don't do anything that could upset this new utopia and lose it all could be wearing you down.

It might just be that it'll take you a while to start to feel secure again in your new position, where you begin to feel like you're a valued member of the team and that you won't be the first to go if there's something you do wrong. To start to let your guard down.

A lot of regaining some sense of job security would involve seeing how the new company deals with other people who work there, do they give second chances, how much do they work with the other employees to resolve situations, what kind or repertoire you build with your bosses etc.

Anyhow, something to think about.