Why don't you just take a break from all the "work" riding for a while?
It's coming into winter, why don't you just do some trails and not worry about your horse being perfect? Work on loose rein stuff without a contact, practice seat and leg responses so you stop thinking of your hands the way you are.
I do get in a way where that is coming from. Learning to ride I want to a riding school that, well it wasn't the greatest I guess. It was very much "pull to stop, pull to turn" - which I think is rather common. It was very much "the hands control the horse". Even after years of better lessons when something isn't working I feel like grabbing the mouth for control, or doing something with my hands. I think it's pretty human though - well feel that if we grasp something, we can control it. Its taken a while for me to really, back off the mouth only using what I need and then releasing.
I think Rookie is right though about this being a temper issue, or a frustration one. You have an ideal in your mind and when it doesn't work that way you get upset, then you are hard on the mouth and then you probably get more upset because you know it's not right but then you do it again.
I used to have a temper, be impatient and frustrated and it's only in the last couple of years I've kind of "conquered" it. I'm still sensitive and touchy at times but I have my way of dealing with things. I'll share how I did it.
The first thing for me was really knowing that there was always a way out. In an argument I can just turn and walk away, with a horse you can always just get off, or just go back to a walk, give them a loose rein and walk out. Sometimes in our lives we're told that if we walk away from an argument we're weak, or the "loser" or you always have to win, or you always have to stand up for yourself. It's not true. As soon as you're in a situation where your emotions are influencing you more than your mind you've lost control. You've lost. You might win the argument, you might fight the horse and win but you've lost control of yourself. To me that's far worse than losing to anyone else.
So to start with whenever I felt myself getting really angry or frustrated or my emotion was taking over I'd leave. I'd go out for a walk, sometimes in the middle of the night or whenever and just chill out. Lock myself in my room, have a shower, whatever. The more I did that the easier it was. I had a choice still - either give control to my emotions or choose not to and walk away. I guess I learned that I was letting whatever was happening around me influence me inside, it was my choice, I could always choose something different.
Once I'd established that in my mind I didn't have to leave, if I was in an argument I'd let my emotions grow to a certain point, then I'd either choose to leave or stay, and if I chose to stay I just shut it all down as if I'd walked away. Or on a horse and I was getting frustrated I'd just "let it all go".
Now if I'm in an argument and they're getting angry, I'll get angry to a point and then shut it down, and the other person will make a complete fool of themselves. If I'm impatient waiting in line for hours I'll get to a point then I'll just accept it and shut it all down.
The other day trying to load a horse someone remarked on how patient I was, where as other people were getting frustrated. I was really happy with that because it's taken me so long to get to a stage where I never get really angry or anything. I'm no zen person, most of the time I am a touchy, over sensitive person but while I might get annoyed or whatever it never overwhelms me to the point I say or do things I'll regret.
So I guess my advice to you is to think about your reactions to things, like being disappointed, or whatever, and work out if you feel out of control with it. If so, find something that works for you. Stick at it and stick at it all the time until you've worked it out. Human's are a like horses, you can't just change something sometimes and expect it to stick
Change takes time and work, there are quick fixes.
Just my long winded opinion, take from it what you will. Don't beat yourself up, you can't change the past only the future.