How do I deal with intrusive thoughts?

The easiest way to deal with intrusive thoughts is simply: Lord Jesus Christ, have mercy on me. Make the sign of the cross. If the thoughts keep coming, we keep praying. That's it. Don't converse with thoughts. If you begin to converse with them, you end up in the thought tornado, spinning around and around, having conversations with people who aren't there, imagining every diabolical iteration of a situation.

Focus your mind on Christ and turn your attention away from what's bothering you. As St Silouan brilliantly said, if a thought troubles us, we're free to leave it and find a new thought. God does not want us to be plagued, burdened, anxious, or weighed down with thoughts.