First ‘big’ job on Monday…not sure I can do it

First ‘big’ job on Monday…not sure I can do it Hey everyone I am 30 and because of a mixture of anxiety and my stutter, I have never really gone for my dream career in law. I have instead worked dead end hourly paid jobs with no intention to stay in them. However, last month I beat a few hundred people (their words not mine lol) to be offered the chance to work in a criminal defence firm as a paralegal and have the chance to progress to be a solicitor in the next few years (I’m in the UK). The problem is that I have given up law opportunities in the last (although none as big as this one) and I don’t see how I can go in on Monday. For example, the interview for this job had to be conducted over video call as I got to the front door and couldn’t go in due to anxiety (I lied and told them something different). The speaking on the phone is a nightmare due to my stammer (it’s kinda mild/moderate but worsens when I’m nervous). They know about it and said it’s ok but I just wanted any sort of words of encouragement or advice that anyone can give. This is huge for me, it feels like a make or break moment. My anxiety is urging me to run far away - my rational brain is trying its best to keep me here.