Two questions Jm:
--what is your stroke rate for 25 meters? Above 20? See paragraph 1. below, from the article "Swim like a Fish". High stroke rate equals high energy output equals "breathing heavily the entire way".
--do you swim schlumpy? In other words, is your torso rigid when you swim or more like a partially deflated condom? See paragraph 2. below.
1.To propel yourself through the water faster you might, for example, increase your stroke rate. But there's a problem here, Touretski says. You'd soon run out of steam. He cites a passage from his favourite book, Fish Swimming by zoologist John Videler of the University of Groningen in the Netherlands, which states that energy consumption in water increases at the cube of the stroking rate. In other words, doubling the speed at which you move your arms through the water takes eight times as much energy.
2. He is known for using unusual props to get his ideas across: he once brought inflated condoms to the pool to show his fellow coaches the importance of maintaining a rigid torso while kicking forward. When deflated, the condoms flopped in the water; inflated, they skimmed across the surface with just a light push.
That being said it's hard to determine what you need to improve without seeing your stroke. Post a video.
Also, as you know, swimming is technique intensive. You want to improve from an average 2:19/100 meters to 1:48/100 meters. That would be huge. I would suggest small, doable, incremental goals at shorter distances, 250 meters/500 meters/1000 meters/1500 meters, etc. --mjm