My DS9 is afraid of thrill rides and big water slides as well. This summer he went on his very first roller coaster and loved it after several failed attempts prior to this. A few things seemed to help him. He has a fear of heights and we discovered this was one barrier. We made a plan for him to close his eyes on the ride and used distraction to get his mind off the fear. We talked about his favorite interests during the wait in line and went on the roller coaster when the line was the shortest. He had less time to think about it and not a big deal if he did not succeed since the line was short. He watched the roller coaster before hand several times and timed how long he would be on it. This helped him to think "I will be in misery for only so many seconds if I don't like it. " We used a reward too as a motivator for conquering his fear. These strategies seemed to work for him and he finally got on the ride 20minutes before the park closed. Fear is so difficult and they have to work through it at their own pace. Not sure if this helps but this is what helped my DS.
