Can I offer a few tips? If you don't want them then don't read on, I won't be offended (or even know!)
If you don't get on a horse & get a bad feel from him then ask to change. The horse may be well within your abilities but sometimes there's something you can't describe that just doesn't click & if you're already nervous you don't need the extra worry. If you explain your problem then your RI should understand, if she doesn't then maybe change her as well as the horse!
Focus on the positive, not the negative. Inbetween rides play back all the times you've enjoyed riding, times you've felt safe & in control & do the same just before you get on. Picture yourself riding well with a smile on your face & a relaxed posture. Don't focus on things that went wrong or you'll just get more anxious that they'll happen again - learn from them in your lesson & then move on.
If your RI asks you to o something you feel uncomfortable with don't be afraid to say no. Over a period of time with good experiences you'll eventually decide to do those things, or if you don't it's because they aren't things that matter to you. Remember you ride for your own pleasure, not your RI's ego.
Learn how to look for positive things, no situation is so bad you cant take something positive from it. A horse played up & worried you? Yes you could keep looking at it an increase in your mind how bad his behaviour was & how worried you were OR you could say he was a cheeky oik but you stayed on, rode through it safely & got him to do what you wanted & if I can deal with that how much more can I do? I'd often feel my most confident after the shit had hit the fan & I realised I could cope, and I'm sure I'm not alone in that.
I hope you can work through your worries x