Searched the forum and nothing good came up.

Say I'm running a 5k. First mile is fine, second mile I try to keep pace so I step up the effort, but probably slow down by about 5-10 seconds. The final mile if I want to keep the same pace I have to really hurt myself, and I just can't do it. I have a "pain barrier" that I can't seem to break. I'll get to the top of my pain threshold and just go into cruise control. I scold myself after every race but i just don't know what to do. It's worse if someone passes me. I'll just let him go, which makes me slow down even more so I won't be 'too close' to him or leave the possibilty of kicking him down at the finish (more pain I guess).

Anyone might have any suggestions?

It would be instructive to consider long term training history. We would all like the comfort of being able to accurately predict race results from workout times (and places) but one workout in isolation will omit much of the story. There has to be some reason why you can outperform the teammates in the intervals but get dropped in races. Possibilities:

a. You're the team "workout king" who treats every repeat like a mini-race and therefore sets unrealistic expectations of what pace you can maintain when the rest periods get removed. If you are running the workout at 3k race pace and your teammates are running at 8k race pace, they will have less difficulty stringing together a consecutive effort because they have much more in reserve.

b. Some people do great in interval workouts because they respond very well to rest breaks every 1-4 minutes. Again, could be a factor in setting up unrealistic expectations if you are in great interval shape but haven't filled in the gaps to be in race shape. This factor is probably some physical and mental hybrid, but it is usually the mid-distance specialists (1500m-3000m) who can feel comfortable ticking off the reps yet struggle when the races go beyond 10 minutes. 5k pace may feel comfortable when you only have to do it for 2-3 minutes. Different story when the rest period goes away and you have to run for 15-20 minutes straight. Which leads to point c...

c. Training history. Have your teammates accumulated more lifetime miles/more extensive training history? Any injury interruptions over the years? Where are your tempo runs? Long runs? What is your overall mileage? These factors all contribute to whether you can sustain a pace close you where you run your intervals. X times Ymeters in Z minutes doesn't tell us everything. The interval workouts are part of the puzzle but you need the tempo runs, long runs, and overall mileage to provide the underlying strength to hold your speed over a long continuous effort. Your 17:15 teammate, for whatever reason, has probably developed his overall aerobic ability to a greater degree and is simply not suffering as much midway through the race.

Look at the bigger picture and aim to reach the point where your underlying fitness is such that 17:15-5k becomes no big deal. You ran 17:35. An 18:30-5k effort is less than 100% of your max 5k effort. Do think the 18:30 runners are thinking, "Man, I wish I had the balls of that 17:35 guy up there. I tried to stay at his pace but didn't have the balls to hang."? Of course not. You are fitter than the 18:30 min guys and the 17:15 guys are fitter than you.