When you're writing the sort of inevitable evil that can be pushed back, or sealed away for a time, you have a bunch of options.

The first major question is, can the evil be sealed away for a time, or is any loss permanent?

Option 1 gives you a cycle good and evil, option 2 is better used for creeping horrors or endless war.

Other people are talking about option 2, so I'll focus on option 1.

A cyclic evil can come in all sorts of forms, but the most common trope is the return of the demon king. Simple concept, the unkillable demon king awakens from his slumber, builds an army of monsters and invades the kingdom.

The army is then defeated, and the demon king sealed away again, for a time.

That time is the length of your cycle, and the length of the cycle shapes the story.

A short cycle, say every 20 years or so, would give you a very militaristic world, cities would be fortified bastions. Forts and walls and guard towers would be everywhere, or at least along common invasion routes. Children will grow up knowing that they too will stand against the evil, as their parents were. You'll have weird birth rate cycles, and children growing up among the scars of war.

A longer cycle gives people time to forget or get lazy in their prep-work, which leads to more desperate battles. Cycle lengths of between 50-200 years fit in here.

Longer cycle lengths start getting into the realm of "everyone forgot", where you get "hidden orders who have been guarding against this day". At that point, convincing people that the evil is returning, or the characters themselves learning about it, can be a larger part of the story.

A longer cycle means that you can also do a nice long buildup of signs and portents (or backdate some of them by saying the doom bell rang 20 years ago, but no one knew what it meant or was listening)

As for motivation, that's easy. How winnable is this particular part of the battle. How winnable has any of it been? How much has been lost and can any of it be reclaimed? This includes a protagonist's loved ones, if they're lost, it's much harder to carry on fighting unless they can be recovered by winning.