12 days ago i was cutting onion and then i suddenly experienced in my eye the most painful eye feeling i've ever felt. ever since, there's been a problem with my vision. my right eye has a spot that's darker and slightly more blurry. will it heal?

See an eye doctor for an urgent visit. Onion juice may of course cause some temporary eye irritation and watering, but I would worry about a foreign body in your eye, causing corneal abrasion or actual foreign body impeded within your eyeball. Foreign bodies may have occurred at around the same time your were working on the onions! You didn't mention whether you wear contact lenses. Good luck!