If it "never goes that way," you're probably setting goals that aren't realistic. My splits for my 8K PR (25:30, ~5:08 pace) were 5:09, 5:13, 5:10, 5:07, and 4:52 (the splits were full miles, so last one was shorter than a mile and very slightly downhill, so it's probably equivalent to ~5:06 if it were as flat as the other miles). Pretty much all of my PRs have been even splits (except for the half-marathon, because of hills on the course), because I knew what to expect coming into the race and I set goals that were consistent with my training and previous races.
If you set a goal and you don't achieve it, don't blame it on how you ran the race or anything else (drinking the night before, etc). Even if there's some truth to that blame, the biggest factor in play here is your fitness; a night of drinking and/or poor pacing will get you to 28:15-28:20 if you're in 28 minute shape, not 29 minutes. Accept where you are and base your expectations of what your training suggests is possible, not what you'd like to be possible.