First (and I'm sure you'll hear a lot of this...), don't ever let the idea get into your head that your bit will give you more or less control or safety on a horse! That isn't any kind of real control, and not a relationship you seem to want with your sweet girl. :)
Here are a few notes that might apply to your bitting situation. Sometimes loose ring snaffles will pinch at the corners of the mouth as the ring rotates, causing discomfort. Additionally, with stronger force, snaffles are thought to have a "nutcracker" action against the palate that can be uncomfortable--some horses are very sensitive to palate pressure versus tongue, poll, etc pressure. Both of the issues can be alleviated with the use of a double-jointed ("french link") eggbutt snaffle-- here is the one I use Korsteel French Mouth Eggbutt Snaffle - Eggbutt Snaffles from SmartPak Equine
. My horse loves this bit and will gladly play with it all day if I let him! I always recommend this bit to everyone I meet. When you apply pressure, the "dogbone" in the middle will apply pressure to the tongue instead of bending up to push on the palate; it is a very gentle and fair bit, I think!
For the love of goodnessgracious-- please don't use that twisted wire snaffle!
I'm sure that you can only imagine the kind of havoc it could wreak on a horse's mouth in the wrong situation!
As for learning to let go of that fear... something that a lot of people do in order to get used to going fast on a horse (learning to sit the canter safely and comfortably) is to have a trainer or experienced horseperson lunge them on horseback. This will give you the chance to feel what it is like to ride that kind of power and speed, but without the worry of "are my reins too tight/loose, am I in control, will the horse run away with me?" If you don't have someone available to lunge you, try cantering in a smaller place such as a round pen; I don't love fast work in a round pen, but right now I think the most important thing is to build on your confidence!
It's all mental. :) You literally have to tell yourself to sit back, give a soft but firm cue, and "throw" your hand forward (I put that in quotes because in reality, you aren't throwing your hand anywhere-- but you are resisting that urge to yank it back and bend over it in the fetal position!) and focus on relaxing your hips (...or as a 4H'er at my barn says... moving your hips like a belly dancer!). If you trust your horse not to do anything crazy, there is nothing wrong with not giving her direction but letting her canter big old circles/oblong ovals/triangles/whatever around the arena while you focus on breathing deep! When you need to pick her back up, she will notice it and then it is all a matter of timing and balance to make the downward transition from canter to trot, walk or stop a smooth and comfortable thing. And, once you start to feel comfortable riding a faster pace, you can start to incorporate leg and rein to direct your horse-- for example, feeling your horse lean into a turn and picking them back up by scooting their hindquarters back under them with your leg.
I hope that this helps! I think you are hearing lots of fantastic advice and that there's more to come!
Edited to add... two things! Don't be afraid to use your hands on your saddle to make yourself feel comfortable. What I mean by this-- don't worry about how it looks if you have to reach down and hold your horn to keep yourself from bracing on your horse's mouth! And (once you feel comfortable enough to reach back), don't be afraid to reach back with one hand and grab the cantle to literally pull your butt into the saddle to ride a seemingly too-fast, too-scary canter that in reality is just a little more fast-paced than you know how to ride! You'll learn the confidence quickly, and the motion over time!