In my experience willpower is not sufficient for behavior change. It’s this false premise that people tout as a solution to all problems. For example, you want to lose weight, so just use your willpower or motivation to eat better and exercise. You want more money, use your willpower to motivate yourself to work more. It turns out that doesn’t work.
To make life changes - to work more, eat well, exercise, stop smoking, etc - habits must be changed. Habits tend to be subconscious behaviors and drive the vast majority of things we do and decisions we make. If you want to work more, you need to think of it as a habit problem, not a motivation problem.
There are many people who can do a better job than me at explaining how to build a good habit to meet your goals, but the short version is that you need a few things - a cue that makes you “automatically” start the behavior, a situation that enables the behavior (makes it easy and eliminates barriers / excuses), and an immediate reward for the behavior (a paycheck isn’t usually immediate enough).
So if you really and truly want to work more, you need to make it a habit to work certain times that add up to the number of hours that you want to work. And you need to reward yourself for working if the work itself isn’t rewarding. The nature of the flexible schedule makes this harder.
I recently listened to a good podcast about this (I listen to podcasts as a “reward” while walking the dog - which makes that a strong habit):
https://hiddenbrain.org/podcast/creatures-of-habit/Or if you prefer reading, these are really basic, but give you a good idea on how to create a habit:
https://www.npr.org/sections/health-shots/2019/08/16/747332849/fresh-starts-guilty-pleasures-and-other-pro-tips-for-sticking-to-good-habitshttps://jamesclear.com/three-steps-habit-changeYou should also consider if you really want to work more or if you just think you “should” work more. It might not make sense to work more for $14 / hr. Maybe that time is better spent getting a skill that would result in a more interesting job that pays more and that you’d feel more “motivated” to do because the work is more intrinsically rewarding.