50m fatigue

So I've swam my whole life until I was 12, and now I'm 15 and started swimming again 6 months ago (seriously training - I didn't fully quit after 12). I used to be pretty good for my age, but now I get tired after 50m. I'm super active, I do 2 other sports competitively and they both use the whole body. I also go to the gym a lot. I've been told my technique is great, so does anyone have any ideas as why am I struggling/ could give me drills or a plan to improve?

My 50m time is 36 seconds btw, I've improved since I've started but I stopped progressing 2-3 months ago. I swim 3 times a week for 1-2 hours.