You're not a bad person because you accidentally ate meat, and it doesn't make you any less a vegan. It happens often to many vegans. Brush it off and learn from your mistake to reduce the chances of it happening again.
Yeah, try labeling your food. But also tell your roommates that your tupperware is off limits to them and you're going to start throwing out any food they place in it. They don't have a right to your tupperware and aren't in a position to complain about respecting others' things if they continue using it anyway.
There is a rational form of veganism which states that what matters is what you intentionally purchase. Not accidentally eating meat, not accepting charity food items containing egg or dairy as a person who is poor, not "dumpster diving."
I do see people online who post weird creepy stuff like "he's been vegan seven years, that means all of his body cells are vegan." Like wut? They make vegan sound like individual detoxification instead of a social justice movement. Even from a health angle this is kooky, since eating traces of animal products doesn't really affect things like heart attack risk.
Still I sympathize with being upset. Meat bothers me more than anything, I would never eat lab grown meat even if no animals were harmed. I don't want to eat lab grown people, either.