Willpower is just an abstraction. It is not a “real” thing, consumed once you do something you wouldn’t.

That said yes, you can train it. Or rather, condition your behaviour so you do things that you’d otherwise avoid. The how-to is actually simple:

1. Choose a task to perform. Push it a tiiiny bit harder.
2. Did you manage to push it harder? If yes, reward yourself. If not, skip.
3. Repeat 1 and 2 for some time.
4. Gradually decrease reward frequency, make it a bit random. Oddly enough this makes the desired behaviour to stick further.

Eventually you won’t need the reward, but the behaviour is still there.

What you consider a reward is up to you. For example, for me snacking on cheese is a reward, but it might not be for you. With the right mindset, even mundane things can be a reward, like taking a comfy shower or playing some games.

There’s a very good video/podcast episode by Andrew Huberman called something like “How to improve tenacity and willpower”, and it discusses the idea of willpower being a finite resource. The gist of it is that it is finite if you believe it’s finite, and can be partially recovered by costuming glucose if you believe glucose is that finite resource.

As for whether or not you can train it, yes you can. The same brain region is responsible for this willpower regardless of what the task is, so if you get practice drawing on this willpower in one part of your life, it will improve your ability in other aspects.

Lots of good answers here, I just want to add a book recomendation: Atomic habits. It is a practical way of training your “willpower”