If you want to get better at game mastering; think of the piece of game mastering that you avoid. That thing is probably what you need to work on. Think of the adventure type, the campaign type, monster variety that you actively avoid because it is hard. I’m not talking about how you avoid dungeons because you don’t like dungeons. If you avoid making dungeons because you are better at above ground exploration or investigation then you should be working on making dungeons.

What ever it is you are bad at, work on it. Every day if you can. If it is too daunting or too big of a task you are trying to figure out then break it into pieces and pick a piece that that seems plausibly mastered in a short period of time. Then do exercises. Just make door encounters. Do a half an hour a day, every day for a month. Write them down in a notebook. You can use them later or just look at them and learn from them. When you feel good about that bit, move on to the next hard bit you are avoiding. Exercise the skill, the smallest specific skill you can identify. Maybe that is just a one sentence room description. Write two a day. Or how ever many you can stand. Keep going.

As  you learn each individual skill you can then integrate them into the big skill that is game mastering. After a while, you will be good. It takes time and effort. But it is worth it.