I don't think you have a mag def.. I think it's actually a lock out type situation. What your describing and from what your feeding purp and big bloom from day 1.... You have saturated the medium with salts. Check your tds and ph your run off. Then you should have some idea where to go from there. But do not foliar spray at this stage. Your plant has actually been slowing down for quite some time now. look to your feeding practices.. I'm sorry I know it's not nice to hear for some.. But yes you have indeed done something wrong.. I mistakes all the time.. It's just important to identify your hand in all this growing stuff.. Plants want to grow.