Practice relaxing in the saddle, even if you just lose the stirrups and just be with your horse. Do small things and do them repeatedly (not all at once or you'll bore your horse) until your very confident doing that spacific group of exercises. Then add in a new one to the group after you master your original set. I've figured out that just having a trainer or fellow horseman with you during your practice can be very calming.
Maybe try something you're familiar with every time you get anxious, eg doing a figure of 8 in walk or trot. Maybe Justin stopping and taking your feet out of the stirrups and taking a deep breath? Also try square breathing, in through your nose, hold for 4 counts, out for 4, hold for 4 and so on
Also something that helps is when you think about this: even pro riders like Charlotte Dujardin have started somewhere and been at your level and struggled (and maybe still struggle) with what you're struggling with and that they all had to start somewhere. They all started knowing nothing about horses. Now look at where they are! Be inspired:) That helps me. And those pro riders also still struggle with other things, I bet. :)
For me, I have to pretend that when I'm riding, it's just me and my horse out there. I also have to tell myself that everybody has been at my level before and that I'll get better.
You have to go out there like you've never lost and you aren't going to stop now. You have to pretend you're perfect, even if you're not.
Tell yourself that nothing will go wrong, and if something does, you'll learn from it. My favorite quote is, "I don't lose. I either win or learn something." And I think it is a great quote. You build confidence when you believe this, and you know you *can't* lose. You learn something from what you did wrong and the next time you go out there, depending on what you did "wrong," you will either right it or improve greatly.
You can't worry about all the bad things that could potentialy go wrong. Don't focus on all the what-ifs. Focus on what you can and will accomplish.
I hope this helped. This is some of the things that help me as a rider, especially since I have really bad anxiety about things:)
I have bad generalized anxiety and what helped me a lot was going back to basics and riding a horse I felt completely safe on. Lots and lots of work at the walk and trot, working on remembering to breathe and catching meltdowns before they happen so I can stop and collect myself again. I also took some barrel racing lessons with a very patient instructor and that did wonders for my confidence. Find something that helps you stop overthinking. For me that's trotting spirals or cantering circles, something that will pull my mind off of whatever I'm worrying about.