Stress is a lot like housework. When the dishes, don't get done and a mess is left in the living room, eventually there becomes a backlog that seems overwhelming.
Issues that you haven't resolved in your mind are like dishes left unwashed. When a number of these mount up, they take up brain space and leave you feeling overwhelmed.
You have to list every thought taking space in your head, then resolve each issue, step by step. By resolve, I mean make a decision. Do this, don't do it, do it later or whatever.
It's basically about making a decision where you stand on each issue. Once you've made your decision, your mind doesn't have to hold the thought and so you resolve the stress.
I wrote a series of blog posts on resolving stress that you can read more about by clicking the links below.