Too nervous to resign

I got a job offer from another company and I accepted it. I told them that I have to resign first for 3 weeks to a month.

But It has been 1 week now and I cannot file a resignation letter. I am working on a startup company where I have already made a bond and closeness with my boss. I am to nervous to tell him that I will resign. What should I do?

I got terminated here few months ago but my boss defend me from the human resource.

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