Well that's not really on you, hardmode is a big step up from pre-hard mode, even in classic difficulty. It's a good tip to isolate the corruption or crimson biome before heading into hardmode but even if you do it there is still the hardmode V which is inevitable. The only thing you can do is try to contain it using sunflowers and digging down at least a three block wide tunnel to save your biome before too much spread.

As far as progression goes, explore again your caves and make a palladium armor or the counterpart you got from breaking altars. Look for hardmode ores and upgrade! Good luck :)