thanks to Wahine I discovered the 3+2 pattern. This evens out the sides and gives you some more oxygen than a pure 3-stroke pattern. I use this on most long swims and long intervals.

However, you should have exhaled completely before you come up for air. You lose precious "inhalation time".

Taking a class is good. Because with freestyle, it's almost always a matter of technique if you end up breathless at the end of the lane. I'm taking a guess : it most likely has to do with body rotation and your water balance, arm recovery and timing, timing, timing... learn to love drills.