OK practically speaking you doubtlessly know that you can get extenuating circumstances on the exams and that can be taken into consideration - make sure you talk to your school about how this is affecting you as this can translate into additional marks but you have to tell them obviously!

You may find that there are times when this worries you more than others - late at night for example avoid revising then.

How are you revising? re-reading past notes makes it easy for thoughts to intrude. Concentrate on doing past papers and writing them down. It's easier to concentrate on the work that way.

Its hard at this age but its all about mental discipline learning how to think of something else when the thoughts bully their way to the front of your mind. Try to find something that you can think of or do when it happens. Dont sit there sobbing into your books be tough with yourself, get up go get a drink, break the thought pattern. But don't get drawn into YouTube or something you find it hard to get out of again when you're ore in control.

Best of luck