About lowering stroke count.

I am a novice swimmer. It usually takes 19 strokes and 22-23 seconds for me to finish 25 yards and I can hold this pace beyond 500 yards. Since people always say longer stroke lenth always means better, I try to improve my stroke length. The problem is when I try to finish 25 yards with 17 or lower strokes, I become exhausted very easily, also the speed is even a little bit slower. Now I want to improve my swim conditioning, what kind of stroke should I use? the exhausting, slower, but longer one, or my previous easy, faster but shoter one? This information might be useful. My hight is 173 cm, weight 134 lbs (Damn!). My kick is pretty weak and I can barely finish 50 yards. When I swim, I use 2 beat crossover kick.