Heya,
I think that you should talk to your instructors about the issues you are having. You don't need to be very specific, just let them know what you are diagnosed with and that it is effecting your participation in the karate class.

On the other hand... I am a karate student, and have trouble at times because of my PTSD and anxiety. One thing that helps me is to work with someone who I trust and know won't hurt me. It also helps me to take small breaks during training to just tell myself that this is now, not then, and right now, I am learning to defend myself, so that what happened then can never happen again now. I also take deep breaths to try and calm myself and focus during training.
As for doing high kicks, you don't have to be able to kick that high when you are first starting. Heck, I have been doing karate for two years now, and have a green belt, and I am just barely able to front kick to my head level. My roundhouse, side and back kicks are nowhere near that high, and it's not even worth mentioning my other kicks haha.

If it helps you to prepare though, you can do leg swing exercises to help you to stretch the muscles in your legs in order to make it easier to kick higher. The front leg swing stretch is pretty easy. If you get into a front stance (if you know how to do one - sorry, I don't know your ability levels), you just swing your back leg up in front of you. Keep your leg straight without locking your knee, ankle straight (basically forming a 90º angle between your leg and foot), and keep your toes pulled back. When we do them at my dojo, we do three levels of leg swings - knee, waist and as high as you can - whether that means belly button level, shoulder level, or two feet above your head. Do four at each level before moving to the next level, then do the same thing with your other leg.

I hope that stuff helps, and sorry for the long-winded explanations - especially if it is stuff you already know!!