Reddit mentions of Lister Bandage Scissors 5 1/4" Kits

I think you just need to keep reassuring her. Especially reassure her that even though you were physically uncomfortable and freaking out a bit, you always felt safe with her— that you still trusted her, knew she wasn't trying to do something you weren't OK with, etc.

It's actually pretty easy to accidentally put someone in a position where they asphyxiate or lose circulation when experimenting with bondage. That's why it's a good idea to always have a sturdy pair of scissors (like bandage shears or EMT shears) in the toy box in case you need to get someone out of the ropes in a hurry. And the top should keep an eye out for anything turning blue or purple that shouldn't be. This page has a nice list of cautions for a bunch of different kinds of kink. Jay Wiseman's book is often recommended in the BDSM communities I've known.

On another note, maybe your profuse apology reminded her of her old abusive(?) relationship? Maybe it was one of those things where she always had to apologize for being hurt by him ("I'm sorry I made you beat me!", "I'm sorry I didn't like it when you forced yourself on me", etc)? You don't really have a lot to apologize for after all. Breaking the mood, etc., but that happens, that's normal, an apology is nice but it isn't a big deal. You were probably in an unfamiliar vulnerable headspace and apologized a lot, and maybe she recognized in you the way she acted when she was being hurt.