I have been swimming for 2 years and i'm really good at butterfly. I am really fast and my stroke is good. I have one problem though. Whe I swim butterfly i can hardly seem to manage 50 yd. I am sooooo tired. My coach told me to get my head down quicker after breathing. That helped a little. But i still wanna be able to do a 100 yd fly really fast. Any tips? Thanks