I too was really scared of coasters back in the days, until I rode a few and found out they're a lot more fun than scary. In my experience, there are two sorts of "fears" you experience before riding: The fear of being upside down (inversions), and the fear of heights.
The fun thing with inversions is that they look a lot more scary from a stationary point of view than they do on the ride. Unless you're sitting in front row (and generally, you don't if it's your first ride on a big coaster), the seats in front of you, and the car you're sitting in, will fill most of your field of vision anyway. And those things will remain stationary relative to you for most of the ride, so on-ride it will look a lot more like the world is inverting around you than vice versa. There's also the G forces involved, pressing you against the seat regardless of the direction of travel (which furthers the illusion that you're the one sitting still). Unless the inversion is of the extremely slow type, you don't feel like you are upside down, but rather that the landscape is flipping around. Once you've experienced a few, you realize that inversions is nothing to be afraid of.
As for great heights, they're most intimidating as you're climbing (usually on lift hills), which I still admit to be a little unsettling to me. On the plus side, they usually offer a great view. Lift your gaze, enjoy the scenery, and look at the horizon. Sooner or later you'll go over the top, at which point the adrenaline rush will probably override your fears anyway. The illusion of impending danger will be washed away by sheer thrill. Plus, you're allowed to scream if you feel like it.
There is the "argument of crowds" too. If the rides weren't so enjoyable, there wouldn't be 45-minute queues for riding them. All those hundreds of people ahead of you in line have decided the ride is worth waiting for, because they know it's going to be fun. The park is letting all those thousands of people ride every day, despite getting their socks sued off if anything goes wrong, because these things almost never go wrong. The coasters of the world provide millions of hours of fun every day, and injuries almost never happen (actually, you run a bigger risk walking from the parking lot to the park entrance than you do riding the rides). The thrill rides are engineered to be extremely safe, to be as fun as possible, and the entire business plan of the park hinges on people having a good time riding them. It's pretty safe to assume you too will have fun riding.