You just have to keep working at it. I learned in 8th grade that doing workouts at goal pace doesn't help you. You have to do workouts at your current mile pace, and shorten the rest slightly. I would say you do 8x400 at 78-80s with equal time walking rest. Don't jog a whole 400 between your reps, because that gives you too much rest. Also, what someone else mentioned, do even splits. You haven't broken 5:20 yet, so start out with an 80, maybe do another 80, see if you can keep it up for another 80, and then start speeding up. What you need to do is keep your sub-5 goal, but put it aside. My goal is to run sub-4:20 in the mile, and that's not going to happen for another year, so I set smaller goals first. You should do that.