100 Freestyle Endurance

Hey everyone, I am freestyle sprinter and recently I've been trying to improve my 100 free time which I've done (dropped about 5 seconds) but I need help with one issue in particular. When I swim the 100 I like to swim all out from the start instead of pacing myself at the beginning then trying to turn it on near the end. I find myself starting to drag halfway through the 75 so by the time I'm on the last 25 I am extremely exhausted. I still get good times though! My question and perhaps there are some sprinters here that can help me with this is.. are there any workouts that can help with maintaining the last half of a 100 freestyle? I really need to build endurance for this. Also, perhaps my breathing is messed up? I prefer to breath no more than two times the first 25, no more than 3 times the 2nd 25 and I use the 3rd 25 as the length to get whatever needed air I need so I can keep my head down the last 25 as much as possible and bring it home. Any suggestions would be a great help!