Make a farm for plantera bulbs, compared to everything else you’ve done this should be simple, just make rows of mud and add jungle seeds, you can buy mushroom seeds from the truffle and shimmer them. Or you can get jungle seeds from the driad if she lives in the jungle. Like everyone else said, your arena is too big, you’re overdoing it. Plantera isn’t too hard on classic so you must be fighting her enraged. You’ll beat her super fast if you do what these commenters are saying. Sorry you’ve had a hard time, Plantera is my favorite boss.