I used to be a competitive runner -- my best 5k is 17:10. For years, I was able to train hard, push myself, and rarely felt like running was ever "too hard".
In recent years, running has become more and more difficult, but mostly mentally. I get discouraged by even the most mild discomfort, and take walk breaks often. I still run a decent pace -- and can sometimes run with ease. But I just don't seem able to push past the discomfort like I used to be able to. Then, I get discouraged and the cycle continues.
Are there some tricks I can try to get past these barriers?