From what I remember in Denmark, and this is going back about 14 years, they had us do shuttle runs until we nearly passed out. Also, try playing singles against a double team. The idea is not to run yourself into oblivion, but to exert enough pressure on your opponent to dictate the rally so that you can hide your weaknesses, in your case, footwork.
Iirc, Poul-Erik Hoyer Larsen (look him up) played the All England MS Final in '95 with a sprained ankle and won. He could barely move and played against the world nr 1 a the time. Dictate the match enough (yes, easier said than done) and your weaknesses don't matter so much.
Also, buy some ankle weights from Amazon and train with them. Makes your legs much stronger and increases your speed.
And keep at it. Watching training videos is one thing, but honing your skills is going to take years. Badminton is stupidly deceptive like that. It looks so easy, but it's incredibly hard to master.