I'm going to completely lose it with a lady who tried to cut in front of me twice at a large grocery store
Why do you want to lose it on this rude lady?
Because she's pissing me off and I want to teach her a lesson
I get why you are upset, and it's rude to cut people like she did. That being said you need to show patience in public and not do anything that could make things worse.
Why the hell do you care so much about what I do? It's none of your business. I'll punish her!
I think it would be more wise to just take deep breaths and ask her instead of blowing up. It pays to show respect to everyone, especially in public spaces. How would everyone else feel if they saw a big scene that could be dangerous to them too?