I've been swimming for about a year and a half. I've been able to bilateral breathe that whole time, but I struggled with the whole "out of breath" thing for awhile. I also mentally struggled with the idea that all of my cycling fitness just didn't translate to swimming fitness. For me, it was simply getting enough time in the pool. Drill and stroke tweaks helped a little, but mostly it boiled down to practice and becoming more comfortable in the water.

A year ago, I wouldn't have believed that I could do a 2000-yard workout and not die. Keep at it, it does get easier.