Running a even pace

I;ve always been told that distance runners should run an even pace and a slight negative split. My problem is that I always run too much of a negative split. If I time myself on the track each lap is faster than the next
when I try running a even effort. If I force myself into doing a even pace for a certain distamce at the start I usually will run out of gas before the end of the run. I always seem to think that i could have run a faster race if only i could have got off to a faster start. Why could this be happening and how can I run more even?.