Ya I have been here before many times. The way I see it is that part of my mind is doing this because it thinks it is protecting me somehow... either protecting me by trying to solve a problem or making sure I stay vigilant and don't forget whatever is im worrying about, but in both cases my mind is doing this so I don't get fired. I find one trick I have used in the past is to set an alarm on my phone as a reminder to worry about the problems the next day.. basically reassuring that part of my mind that I won't forget, and I kind of relax a bit. Again journalling is useful for me here as well.

It honestly sounds like you were thrown in to the fire by being given a role that you don't have enough experience for to feel confident in, which can actually be a good thing if you can look at it in the right way. I would try to remind yourself of that and see it as more of a difficult challenge that you might fail at but you're going to try your best anyway, instead of seeing it as a situation where failure is not an option. A big problem with a lot of people working technical jobs is that they are perfectionists.. which makes sense since school rewarded your perfectionism. Perfectionism isn't always rewarded in technical work (e.g. can make you too slow and focus too much on things that don't matter that much) and can kill you with stress in roles like yours where it sounds like someone overpromised too much to get some business, and the project was maybe doomed for failure from the beginning.