OH hun, you sound so like me, motorways are the worse for me, I just hate it. Sometimes I will get on the slipway and start to panic just thinking about it.
I amazed myself driving from Yorkshire to Dawlish to go pick up my future hubby so I had a great motivation and managed it.
What I did was play nice music and keep my shoulders relaxed, breathed steadily if I felt one coming on, and kept mostly in the left hand lane as overtaking really scares me to panic.
I know people without the disorder that hate motorways, must bring the worse out in everyone. Then you get those guys who couldn't care less and act totally relaxed about it, I wish I were like that.
I used to be really assertive and a good driver but the panic is another thing all together.
I fully understand your problems. Make sure you have eaten a light meal before going on there.
Good luck
lotsa luv
Dawnx