Being unrelenting is a powerful skill. But your willpower is not reliable. You run out of will. Instead of “forcing” yourself to do something, here are two strategies to improve performance.
The first is to understand why you want something. You want to have a pull toward some positive goal, not a push from a negative goal. It’s just a different way to look at it, but it’s essential. If you’re doing something to get away from a negative, as soon as you improve to where it doesn’t seem too bad, you’ll backslide. You want a pull directly to good. Is your goal toward a good feeling or away from a bad one?
The second is to learn how to build systems. You need help to create what you want. Rather than trying to force yourself, do what works best for human animals. Build and follow systems.
Whenever you have to force yourself to do something, there’s a problem with the environment that can be fixed.