My advice for new GMs is not to worry about the endings, or climactic encounters, of their adventures and campaigns. I tell them to focus on setting the stage and engaging with the players to develop things organically. There's three reasons I give this advice. First, GMs need to be focused on the moment and if they are thinking about the ending and not the present they will be less effective. Second, and related to the first, is that if a GM has an ending in mind then they will in some fashion railroad the PCs towards that ending. Third, you often have an opposite effect to the second reason - players will do unexpected things and a lot of times you are wasting time if you spend it scripting a dramatic ending.

This good advice, yet just like with books and movies the ending will shape reactions to the whole package. You can run a fantastic adventure but if the ending misses its mark that's what players will remember. This isn't something I tell new GMs as stressing about the ending doesn't help. This is true whether we're dealing with law and politics adventures or a dungeon crawl - you shouldn't obsess about the ending, try to get there naturally, but when you do get there you need to stick the landing. This is daunting and not always easy.

What makes a satisfying resolution to an adventure or a campaign?

It's not always a happy ending. The essence of a good ending in a roleplaying context is one that resonates with the players, that makes sense given their actions and the setting, yet plucks the right heart strings. Clarity is important, don't end on a wishy-washy note - be decisive and strong. If the PCs have thwarted the traitorous Duke and have evidence of his treason to give the King, then have the Duke executed or assassinated.

Endings are a cooperative effort. Get feedback from the players, ask them what they think should happen. Sometimes you can even ask them to narrate the concluding event, which you should summarize as GM after they have worked it out. For a lot of players, something isn't "real" until the GM says it is. Sometimes you will have a clear vision of the right ending and you should follow your instincts, just make sure you aren't going in a direction that jars with the vision of your players.

This is one reason I believe GMs need to communicate with their players about the tone for an adventure or campaign; this is essential to get everyone on the right page. For example, if you're playing Fiasco it really helps that everyone knows ahead of time and accepts that the session will end in a literal fiasco. Even in campaigns I find it useful to give players a heads up sometimes of what to expect. If there is a big battle coming up and as GM you have set up a really difficult scenario then tell your players, give them a warning. The endings and resolutions that fail are often due to a disconnect in tone. If the players expected a fun swashbuckling adventure and in the end the GM's villain brutally slaughters a few PCs after bringing ruin to their friends and family - this is jarring and rightly bothers players. If everyone is on the same page at the beginning it really helps avoid these disconnects and can encourage some excellent and innovative roleplaying along the way to the ending, whatever it may be.

Players do not like it when their PCs suffer or fail. Some players handle this better than others, but it's almost universal to say players are emotionally invested in their PCs. If players have a heads up that something bad is coming then they can better deal with it and embrace it. On the flip side, if a player heroically sacrifices their PC, don't backpedal that sacrifice because you feel bad for the player. Embrace the player's sacrifice and strong contribution to the narrative.

So what can you do when you botch an ending?

The best answer is to just move forward and focus on your next adventure while mentally filing away the lesson learned from your mistakes. Sometimes you can retcon situations or make adjustments but usually you just need to keep going and not obsess about a flawed ending.

The final point I want to make on endings is that you really shouldn't stress about them. Yeah, you are likely to botch endings that undermine the entire adventure you ran. That's what happens from time to time and when it does it's usually because you experimented and took a chance, which is great because that's how you learn.