I am primary now, but used to be secondary History. I found the best support was to demystify the exam questions and really really thoroughly prepare them for the types of question, the type of answer and so on. I would do quick starters asking them to identify the question, how many points, main arugment and so on, untile they were confident about what would face them. Sounds trite, but familiarity with the papers helps enormously.
Quizzes, fun games, revision fun activities. Timetables of what you will be covering and when, revision sheets prior to the lesson to help them come into it empowered already.
And lastly, lots and lots of you saying positive things, and empathising without making too big a thing of it. We all know that GCSE results are not the end indicator of potential. I sometimes had to accept a student "backing off" in my subject to get the maths or whatever, and that is fine. Reassure them.
I found lots of peer marking helped too, as well as them marking their own test questions with the actual markscheme.
I had an open door policy for weeping, a chill out corner and every now and then would just go "Blow it! Let's boogie!" and put some music on for 5 minutes.
I am now using all these tactics with my Year 6's, which just breaks my heart. At 11!