Do You Swim at One Speed?
Try mixing in some Faster swims with Active Recovery swims in your training. Power Strokes might mean stronger strokes or faster tempo strokes (think: racing!), but either way they will get your heart rate up and they will probably feel unsustainable. Active Recovery strokes will be easier than your normal pace and will allow you to catch your breath and slow your heart rate while swimming. If you are a new swimmer, you may need to stop to catch your breath, but as you gain endurance you should be able to learn active recovery (it's an excellent thing to practice in the pool as well).
Try This (feel free to modify):
Start out with 20 Power Strokes and then immediately 20 Very Easy Strokes. Did you catch your breath?
Try 40 Power Strokes and then around 1:00 Active Recovery
Now try 60 PS and 2:00 AR.
See if you can simulate a race situation where you need to settle yourself and then speed up again:
80 PS/1:00 AR/2:00 Build to fast (sustainable)