The new job means new responsibilities, new relationships to form, and the desire to prove oneself. It is natural to be nervous in this situation. Most employers recognize there will be an adjustment time and do their best to make you feel at ease. Despite that, there's always that initial nervousness.
I've theorized in the past on the challenges of adulthood. The popular example is making friends as a child. You simply walk up to another child and ask if they want to play. As children we don't really feel bad if someone answers "no." We move on. As we age, we learn social customs and more importantly what does not work. I cannot punch you in your shoulder to introduce myself. So the older we are the more rules we have written inside our head for friendship introductions. It's hard to make friends as an adult as you try to avoid the mines you've stepped on before. Starting a new job brings up all the lived experience in my past of "things not to do" and my anxiety wants to run away with that information.
Thankfully, I haven't had a lot of turnover in my job history, at least after university. However, I did breakdown mentally. I haven't worked in an office with others in a long time. So the level of initial nervousness on the first day is exponentially increased. My inner monologue that "I am not enough" is a deafening air raid siren. I am biting tears before they flow.
As I said above, there is no one here out to get me. They will onboard me as someone who is new and needs some training. That is the reality. The pit of my stomach refuses to believe this. My clenched jaw smugly pouts at the idea of success. My brain is a bookie taking bets from the other organs on how I will fail this time.
I have to focus. Here I go. I am not alone. I have all this doubt with me.