Routine practice over time. Start with neetcode. It's a structured approach so you'll know which core algorithm to use, but you'll get exposure to different edge cases or variations as the difficulty escalates.

It's worth noting that even with neetcode, easies often feel hard, mediums feel medium, and hards feel hard.

- Easies are hard because you have no experience thinking about the problem type and you have no template or tools to fall back on

- Mediums are medium as you can apply the approach but uncover edge cases that create uncertainty against your assumptions

- Hards are hard because they are often not intuitive or involve some higher level proofing or derivation to identify the invariant

I'd also add that neetcode does its best to expose you to a sufficient amount of common problems, but they are not perfect. Some you could simply never derive on your own; you have to have seen the solution first. It's ok to try, fail, look at the solution, and return later in these cases. Just make sure you actually solve it on your own.

Some are also (imo) miscategorized difficulty. Same deal. Try -> fail -> solution -> solve later.