I’ve faced all sorts of players, but I always struggle against those player, I’ve always struggled to beat players who are like that since I was 4! How do I actually defeat them?
Get a teammate in.
Learn basics of defending as a team. Reduce the area they can play to. So that your teammates know where he is going to go. So you are going to 2v1 or even 3v1 him.
Doing this will also tremendously help you to get away from defending on pure instinct and to better understand what you are doing and how the opponent reacts to your stance and body language.
Still athleticism is always something you or your teammates have to overcome with a lot of work. Hitting cardio and the gym may be less work in the long run.
Give them space so you have time to react and try to show them outside.
If you are small and play as a defender, your only option is to become strong and have a low center of gravity. Think Kante.
Positioning.
This. if you’re marking someone faster than you, you can either drop deep and give yourself a yard to get in to position to block their run or if last man you can try and play them offside.