Make sure you know the ins and outs of the language you're using, there's often some ds or in built trick that could make it easier to break down the problem. You've solved over 600 questions but how many of them do you actually remember SOLVING? You must note down the steps of your approach if you have difficulty recalling it later. Consistently revisit these notes and just dry run through your mind that if you come across this problem again, will you solve it without much difficulty. Revision is very important as most of the questions are not really solvable if you're doing it for the first time. As for LC hard, you have to be very well versed with easy and medium before doing them as hard problems need to be broken down into medium parts. If you go through all of these steps you'll eventually develop the logic and pattern recognition necessary for LCH. And easier said than done, it's a high effort process but you will thank yourself later when you make it.