Can't do 50 meters without having a stop in freestyle, why?

In breaststroke, I can swim 200m or more without stopping, but I struggle with freestyle. When I try to go slower, it actually feels harder because my breathing is tied to my speed. If I want to breathe more often, I have to move my arms faster.

My coach thinks I should slow down even more, but I find it difficult because I need oxygen more frequently. Right now, I breathe after every third arm pull.

What do you think I’m doing wrong? My cardio shouldn’t be the issue since I also do other sports like running and cycling, and my VO2 max is 57.