Sounds like you tried to flip the lights and didn't get it done. I'm suprised it took 19 days to realize it, not to be rude.
I need to know:
How long you vegged?
How big is your pot?
How big is the plant?
What kind of light you're using?
What you fed it during veg, and during the "19" days of flower? That was actually almost 3 weeks of extra veg.
Answer those and I can give a better answer. My suggest is to either keep it at the 18/6 light cycle, flush it, and continue on veg nutes until ready to flower.
Or flush it, flip the lights to 12/12 like you tried to do, and then start feeding, and water/water/feed in that cycle.
Either way I suggest flushing because of the screwed up light cycle with unneeded nutes at certain times might cause lock up of other nutes. Flushing just starts you from fresh and seems like a better solution to me.