Help needed please!!

I didn't know where to post this..sorry. I'm 18yrs old and have been told my body type is dense in water. I have great genetics besides the fact that I'm 5'4, I use to play football up until i started to wrestle in 10th and 11th grades, body build, and now I just run and do calisthenics so I have great endurance. I'm training for AirForce CCT and I need to be a proficient swimmer before this September. I can't even swim 150m. I do 50m and am out of breathe and my muscles fatigue quickly. I have good technique I think and its most likely the breathing that does me in. I start off breathing every 5 strokes than tire out to every 3 strokes. I read online that I might take in to much oxygen and not expell all of it which causes a Carbon Dioxide build up over time. After my breath is taken I hold it until after the count of strokes is take then on my turn I exhale all at once like a burst. Am I suppose to exhale immediately, slowly after taking a breath and not inhale more than necesarry? I need help and my goal is to be able to 1000-2000m nonstop freestyle. I need to be able to do at least 1000m before I can leave for CCT training as Combat Diver and SCUBA school is intense. Also in the amount of time from now until September, will I be able top go from where I am at to swimming 1000-2000m nonstop?