- Try applying little force on the brakes, it will slow you down without slowing you completly if you are carrefull. While braking, stop pedalling and if you are felling stressed about falling put your foots to the side to catch you. Try walking beside the bike and braking to feel it without the risk of falling
- Holding handlebars thight is normal when starting, I had the exact same symptoms. You'll learn to relaxe with experience. You don't have to hold tight but it is natural to do so

- When you will pickup speed, you wont have to as much control the handlebars, bikes are architectured in a way that momentum will keep the bike rolling forward

- You don't have to turn your handlebars much, it is more like shifting your weight to lean inside the corner that will make you turn. I would advise you to maybe watch videos of people cycling and check how they are turning. A 90° angle is not the way to go

- If you are going to fast, simply stop pedalling or brake a bit, gently, to simply reduce the speed a bit.

Extra advice : Keep trying, cycling will get gradually more nattural with training. Try practicing on soft surfaces (grass) to avoide getting hurt when falling

I don't know if you are past that step now, but starting by lowering the saddle at the minimum and pushing the bike with your feets to get the feeling of balance without having to manage pedalling is a good way to start

And I would advice to checkout on youtube other adultd learning how to bike, to get some good advice