Breathing

My breathing is just horrible - I could swim all day long as my arms and legs don't get that tired (I have been a distance swimmer all my life), but my breathing prevents me from doing more. I have to take a breath just before my turn and just after and I'm stuck on breathing every other stroke (after about two to three hundered yards I can't even hold my breath long enough for bilateral breathing). Shoot, I can barely complete 25 yards under water without practically passing out. It's not that I am out of shape as I swim five days a week (oh, and I am a non-smoker too). Is there something I can do either in the water or out that will improve my capacity for holding me breath? I know this is a strange question, but I don't feel that the rest of my body is getting the best workout if my lungs are giving out so early. Thank you.