What a great thread, it's good to know so many people, even experienced riders, sometimes are afraid. I'm definitely more nervous about riding than when I was younger, but I can't let it stop me. I've never had my own horse, I just ride whenever i have the chance, but when I haven't been able to ride for a while, i feel like something is missing in my life. And then when I get up on a horse and its been awhile, I'm terribly nervous, and I know that makes the horse more nervous, but its hard to relax at first. But once I'm up on the horse, somehow I start to feel like everything is right, and this is where I'm supposed to be, and so happy that I'm able to ride this amazing creature.
I think you can definitely find the right horse, one who's calm and maybe 'bomb-proof', and i think it helps once you get to know the horse and trust them, and they also start to trust you. Some horses are just too headstrong if you're not very assertive. Maybe a smaller horse is also the right choice, as it just seems a little less high up. Even if you get nervous when you start to ride, maybe try & focus, like some people were saying, breathe to relax yourself, focus on what you want to accomplish today, even if its just a lazy walk around the field, and remember why you're there in the first place, because you love horses so much your fear can't keep you away. Strangely, it also seems to help me a little when the horse does something like spook a little, or even bolt, and then after it, I realize I helped calm the horse down, or just that I stayed on and didn't lose my head, and I remind myself that I'm not such a bad rider. It seems like you're a natural rider, and just your fear or anxiety might be making you a little stiff. I'm sure you'll find the horse you're looking for!