1. Your first sentence sums up a lot of your issues. If your trainer commented that you over think things, "worrying about what to worry about next" must be a pretty serious issue with you.
I'm sure you do that with the rest of your life so, controlling it on top of a horse, as a new rider, is a serious matter.
I have always been this type of worrier, except when I'm on a horse, then I wear a "no fear" t- shirt
2. One thing you haven't talked about, nor has it been mentioned, is connecting and building confidence from the ground. I'm not talking about disciplined drills in the round pen or arena. I am talking about very informal but well mannered "doing stuff" on the ground with her.
Take her for walks, like you would. Dog. Talk to her, scratch her neck, w
If she starts to walk off too fast, gently but firmly slow her down and say "whoa back a little, mummy can't walk that fast".
The horse sounds to be very well broke but on the dominant side. It is your job to fairly let her know you are the boss when she is in your presence. Let the punishment fit the crime. Sometimes a light tap on the neck or nose is sufficient, other times she might need a little more.
You are sweating the small stuff and it will do you in, if you don't find a way to get control, as your trainer has subtlety tried to tell you.
It's pouring much needed rain in your area right now. If your time is yours, get out there between rain storms in your worst pair of barn boots and "handle" your horse.
I am 68 and have not been able to ride since a bad accident in 2006. I still have my horses. I spend a couple hours every day, just playing with them.
You would be surprised what they know and how well mannered they are. If my back would hold together, I would have no qualms getting on them and riding right out the driveway. They might be a bit spooky at things on the road but they stay broke because I know how to keep them gentled from the ground
Find a way to overcome the overthinking and do some things from the ground with your horse. While horses are NOT dogs, you build a bond with a new dog by doing "stuff" with it, the horse is not that much different in that regard.