Sheltie, I really don't have words of wisdom for the anxiety aspect, or any great tools on how to deal with it, I wish I did, but I just wanted to wish you well on your journey, and pray you find peace in the coming days.
I do know what it's like to be nervous, apprehensive, and have anxiety to some degree. To me it's perfectly normal, and to be expected, but to someone that's hurting or in pain over it my heart goes out.... It's not something to take lightly of course. As far as these things affecting the physical outcome of the surgery, I honestly doubt they do. Physically we will heal... and then eventually emotionally/mentally as well... once we calm down by whatever means or reasons.... and that part might take longer for some then others. Hopefully you can calm the waters before your surgery, but if not I'm sorry, I do think though it will still come out fine.
I see you've been there before too, and it sounds like that went very well. For me, I think that's what has me at peace this time, is my last one went very well. I'm going in a day before you(May 5th) for my RTHR this time, and in talking with my wife lately, we're more focused on the future, and now starting to rediscover the possibilties, once again. Those thoughts are really what's driving me, and many more happy thoughts.
I do hope you find your happy thoughts, and they flow like a river to you when you do. That's about the only tool I have in my box right now, and I will look forward to reading you in the recovery forum, and hearing how you're doing. You're not alone, and many of understand exactly what we/you are going through.
I must say too, I'm a huge Sheltie lover! I had mine for 15yrs, and now that I'm finally going to be able to walk again freely, it might be time for another one for us. I've held off because they need walks and activity, but I'd encourage you when you're ready to take that big doggie out for some walks and such. Maybe right there might be the best therapy you can get, for both you and your beautiful Sheltie.