FIRSTLY about the "I think I'm a bad person" part: you are NOT a bad person. You've been a great friend. You were like me a few years back just careful and every, single, word I said. Just learn to be confident in yourself, and know that people are demented and you're not going to be forever imprinted into someone's mind as "the worst person ever". If you keep saying that you're a dumbass that's only going to get worse and exaggerate the problem.
SECONDLY, about the deteriorating mental health state, and this is a repetition of the fact that you're NOT a bad person. You need to learn to let go of things and just live life knowing that not every single decision is going to decide the entire path of your life. You are NOT going to end up as a homeless person on the street just because someone thinks your breath smells bad.
THIRDLY, your gf. Since, apparently, people seem to have mental breakdowns every single day, your gf is also probably going through something. If you don't think you did anything horrible, you probably didn't. Just stay open and patient. You should ask her if she needs time or space, and then give her time and space.
FOURTHLY, you should also talk to irl friends and ask them for advice, since I don't think we here know as much about you as your friends in the real world. Additionally, if the mental health problem gets very bad, you should definitely consult a mental health specialist.
I hope you find this useful.