How to deal with your thoughts

I want to know how to deal with overthinking what happened with me is if I am doing some work my focus will be on future what if I do this and then this so this will happen then what will is so and I get panic i start thinking about worst condition only for example I apply for job i should worried about how I crack job but I am worried about what if I select and I stuck in some problem then office people will judge me . example 2 I don't have salary slip of last 3 months for my last office they hold my salary so after I return my laptop and other stuff they clear in full and final so I am worried about what will I say if the reject me then what should I do the fact is I have not given a single interview till now but think of future what will happen I want to now how I can deal with this pls comment on this