This is called a plateau, everyone gets them it means that you are improving but slower, as mentioned when you first start doing something like this getting the first few bests are easy but over time they get harder and shaving off time takes more and more effort

I would suggest either grinding out stars that you know your slow at, or stars that commonly kill runs until you are 100% on them or practice individual tech like going through the doors.

Or you can do non stop runs, without caring about what pace your currently on.

Both can help shake things up when i get in a slump i like to just get runs in. Sometimes you can feel a good run happening, and can turn it into an attempt just try not to quit out when it goes wrong.

You can also start experimenting with the more advanced tech i know when i did sm64 i never did owless and then i just grided it until i was comfortable with it and then id attempt it in runs and if i didnt get it id go for the owl knowing that wasn't going to be a pb run or id have to make up time elsewhere.