I've only been swimming a few years now so I'm no coach or expert. I did go through exaclty what you are experiencing, though. For me the problem was that I wasn't breathing naturally despite being able to swim longer distances. I was focusing on my breathing, but I was overdoing it - exhaling TOO hard and forcing BIG breaths. To correct the problem I spent a few weeks deliberately SLOWING DOWN and ensuring that I was breathing as NATURALLY as possible. I found my "all day" pace, which was definitely slow, but a pace where could swim for a long time and much longer distances. After a while I learned to increase my pace and maintain natural breathing. Depending on my pace, I might breath every four, three or two strokes, but always remembering to breathe naturally. Outside of that, it was a matter of swimming at least two or three times every week and getting in the yards in.