Trying to fight off negative thoughts and emotions backfires. They will just pop back up like trying to hold a beach ball under water.
Instead, capture your worries in a document, journal, or note. You may find it helpful to jot down stressful thoughts as they occur to you, especially if you feel worried about so many things that you can't even keep track of them. (It happens – especially to us perfectionists.)
This serves a few purposes: it keeps you organized, gives you peace of mind you won't forget anything important, and means worries stay out of sight, out of mind until you're prepared to tackle them. This may be difficult at first, but it gets easier.