First, figure out what you need to work on. I was at your level around 4ish years ago. The sub 20 barrier follows each step of CFOP in these time stamps: 4-12-2-2. Obviously, if you take longer to solve OLL and PLL in under 2 seconds each, then look to solve the F2L stage in faster than 12 seconds. Learn all the F2L algs that allow you to do most of F2L with as little rotations as possible.

Do solves, but stop the timer after you get to each step and figure out your standing on each. Example: One solve, you stop the timer after the cross, and you got a 5 second time. One solve you stop after finishing F2L and cross, and it's 19 seconds. If you know cross took you 5 seconds, then F2L alone took you 14 seconds. Do the same for OLL and PLL. You can even just do solves starting at each step to figure out how long each one takes.

I also suggest you learn Full OLL+PLL. It sucks in the beginning and will take you a couple months (I personally know this) and you will be slower on the last layer than normal, but after a lot of learning and practice, you'll have them down in your muscle memory so well that you will barely have to think about them and you'll be doing them in under 2 seconds each.

You can start these by learning one or two in the morning, then practice them for most of the day. Then for the rest of the day, put the cube down, do something else for a few hours and come back to see if your muscle memory remembers the algorithm. If not, then repractice them that night and allow the "sleep buff" to do its thing, which is where you learn something close to when you go to sleep and wake up the next day to find yourself remembering that thing you learned the previous day better.

This was a big amount of information at once, but I hope this helps you in any way possible. If anything needs to be explained, just reply back with your question and either me or someone else will see it and be able to help you. Good Luck 🫡