IMO the final fight has to be high stakes, hard, phased, and both success and failure need to have mixed consequences, rather than all or nothing. Maybe they beat the BBEG, but lose a beloved NPC. Maybe they claim the Ring of Wish, but it is all out of charges. Maybe they TPK, but arise in chains in some underworld, where they are shown their failure with a glimmer of hope: though they failed, their efforts weakened their foe enough that someone else stepped in to finish them off.

Afterwards, “win” or “lose”, there should be no happily ever after, only a dissolution of the status quo: the future is open ended, but it won’t be the PCs—one way or another—who do the work of building it. Their saga is over, through some form of ascension or destruction, and thosr PCs are no longer available for play: they might, if they survived, cameo in a future campaign, or become NPCs, or even the next BBEGs.

You want to strike a fine line: there must be closure, but it can’t all wrap up neatly. Your players should feel catharsis that this show is over, while making them yearn for a spinoff. I recommend NaDDpod as a live play with incredible examples of this at the end of each of their (now many) campaigns.

Hi! I have run one long-ish campaign and played in maybe two or three more that made it to a conclusion. So, not loads, but non-zero. Here are the useful things I have observed.

1. Out of game, out of character, pick a definite stopping date. It will be much easier to pace out the ending if you know exactly what time you have to work with. It gets players focused and bought-in too, mostly. Trying to let the story dictate a natural end risks things dragging out interminably and the campaign fizzling

2. Talk with players about what they want to see in the last few sessions as everything wraps up. Not saying you can’t have surprises, but plot points they wished had got a resolutions final moments for their characters, old NPCs and locations they want to see one last time

3. Leave room after whatever climax you have planned for one more epilogue session, light or zero mechanical where the players get to send off their characters. This is the player’s show mainly, as everyone is primarily attached to their characters after a long campaign, but you should use your time to tie up loose ends and show how the world has changed on account of their actions. Everyone should have at least one spotlight scene, and, if they’re into it, players should feel free to call scenes with other players, old friends and foes, etc. to get all the closure they need

I feel like those have been the consistent elements on campaign endings that have been satisfying and memorable for me. Good luck!

do you have more details on what is lacking/the problem? it’s a little hard to give advice if we don’t know what the issues are.