I had no one to take me on roller coasters growing up. Both parents had the issues with them, and so I genuinely grew up with a fear of them. At the age of 16 a girl got me on my first coaster. It had a 78 degree drop, went around 65mph, and had 7 inversions. I can still remember the feeling of that first hill. I was so scared I couldn't scream...mouth opened, but nothing came out,lol. As soon as that drop was over, I had a blast. Riding coasters ever since. I still get nervous when I go on them, but once the ride starts adrenaline kicks in and its a blast.
Some people fear coasters because of motion sickness, others because you are out of control. I was,and still am sometimes, the later. I always push myself to ride them though. The first one of the day is always the toughest. With Space Mountain, ask to sit in the first of the 2 rockets. The rear rocket gets wiped around more. Its basically BTMRR in the dark. For Everest, when you get to the part where it will drop backwards do the following...push your body into your seat snuggly, and push on your feet. This is a mind trick, will make you feel more secure and in control. Then as soon as it starts to drop backwards yell. When it stops and prepares to go forward do the same thing again. Don't be embarrased to yell, I guarantee you won't be the only one. It will release your anxiety and won't be as bad. DON'T tense up your stomach muscles...it will make it worse. Lastly, ask to sit in the middle or front...always a little less intense there.
Have Fun, and share your on ride picture with us when you get back, lol