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Some rides will have seats available for you to sit in before getting on the ride. These are mostly to make sure you will fit properly, but trying them out can be a good way to get more comfortable with what you are doing. The best way to face your fear is to push past it and get on board. You've come this far. You've learned about the ride, relaxed yourself, stood in line, and now you are here. Get in the seat, let the attendant strap you in, and get ready to roll. Most rides will have some type of "grab bar" or other restraint that's meant to help hold you into the ride. It might make you feel more secure to grab hold of the restraint. It won't actually make you any safer, but can be a pleasant reminder that something is there to hold you in. When the ride takes off, you will be tempted to squeeze your eyes shut until it's over. Don't do that. Instead, let your eyes guide you along the track, watching what comes next. Being able to see what follows will help make you feel more in control of what's coming. Once you've gone on the ride, great. Now get back in line and go again. Going on rides more often helps habituate you to it, and will make you even more comfortable as you go along.
Try out the seats. Get on the ride. Hold on to the bar. Keep your eyes open. Go a second time.