Wow CAHMS are the worst (I'd honestly say that they ruined my life). But anyway, it's very clear that you need help and I'm surprised your school hasn't done anything? Also, this would count as mitigating circumstances if it's really going to affect your exam performance and you should ask your teachers what they can do in terms of "special consideration" or if they can give you more support. I think your only option (unless you can go private which probably isn't likely) for getting medical help is going back to your GP but try and see if they can do anything that isn't CAHMS. Try not to stress yourself out over the exams because it's only going to make you feel worse.