What's happening to me?

So, this morning we had a 6 mile temp run, but because of how I run, I did a 4 mile instead. I guess it's a habit, because I slowed down quite a bit after my first mile. Since I slowed down, I had an ok run. It bugged the crap out of me though that I go out to fast and then easily die off. I really friggin' want to be able to keep a good pace. It's weird too, because every time before a hard work out, I get a great mind set, I think. I think about what we are about to run and imagine me running really well and feeling great and it really gets me pumped to run, but then BOOM I friggin' mess up and feel like I did a ok run after words....... I gotta get back to doing the best I can, fight through the pain with no prob. Because I did a four mile near the end of summer at 22:59, and today was friggin' 24:35..... I'm so confused and need help! >.<