Hi there. I understand how you are feeling. I've had to go thru this with 4 dogs (well one was a neuter

).
A few things I wanted to offer that might help you here:
*Trust your vet - they do these procedures all the time and your Mitsi will be in good hands.
*Ask the vet office about letting you stay there and being there with Mitsi. This is something
I never knew was possible but have learned from this forum that just about all vet offices will allow this. It's amazing and might help YOU during this time more than anything.
*Also, I've never had a spay/neuter change my dogs' personalities. You have a big part in this, though. Don't act like it's a big deal when you go and when you are there (if you stay) and when you come home. If you act worried and nervous and tense, this may translate to Mitsi having these feelings. I know you will worry and be nervous, but try to portray calm and "all is good" to her
Oh, and one more thing...quit reading stuff on the internet EXCEPT HERE! We are here to support you during this and answer your questions. You don't need any horror stories because it's just very very uncommon and rare for anything to go awry with the procedure.
Sorry..another thing - make sure you do the pre-op bloodwork.