I think this is more about story writing than actual game play. The game play of a point and click adventure drives a story, and you're complaining that since you know the story already, playing the game you crafted doesn't hold the same appeal. I think you can compare this to tickling yourself vs someone else tickling you. When you tickle yourself, you don't feel tickled because you're in concious control over where your fingers contact your skin. When someone else does the same thing, you don't know where their fingers will go (because you're not in complete control) so the anticipation of mystery and potential touch is what actually causes you to feel tickled.

So, how does this work for story writing? When you're the reader of a story, the author is the tickler. You don't know where the story will go, what plot events will happen, so you get feelings of anticipation, mystery, suspense, and wonder. You just don't know what's going to happen next, and that's exciting. If you re-read the same story, you lose a lot of that suspense, but not all of it. Okay, so how do you create a fun story? I think... stories sort of write themselves. You, the author, are just sort of the conduit/medium for the story. You just have to let your fingers rest on the keyboard and let your mind be a stream of conciousness, and write whatever comes to mind, and write it well, and the story that wants to come out, will present itself. The first time you write a story as an author is also the first time you're reading that story, so in a way, it's a unique privilege to be an author because you get to be the first person in the whole entire world to read this new story. What twists and turns will there be in the plot? What happens to the characters? What exciting events will unfold? You get to decide and find out!

When it comes to story writing, I think it's a mistake to look at the finished product of other authors and think that they wrote the story one sequential word at a time. The reality is that a book is like an office building. You can look at the finished product and be amazed at its elegance and design, but a casual observer will never see the scaffolding it took to build that office building. With writing books and stories, you're going to probably be iterating over the same story about 20 times. The first few iterations are going to be you fumbling through the darkness of your imagination, trying to figure out what the story being told is even about. With each successive iteration, you get a better and better feel for the story, and instead of trying to figure out what story to tell, your focus shifts more towards figuring out how to gracefully tell the story you've been practicing over and over again. You can be more like, "I know I have this plot event coming up, so how can I setup the current narrative to nail that plot event?" Eventually, you become more like a musical conductor, coordinating players and their instruments, creating a synchronized dance of words on the page. This process in itself is a pleasurable activity, so if you thoroughly enjoyed writing your story and its practiced variations, and then you put that story to a point and click adventure, you can almost be certain that the game will be amazing.

"In days to come, I shall stand here and think 'I did that?'."

It would be like writing source code, and then returning to it in a years time( or longer ).  Yes, you will still remember the highlights, but there are moments when you fail to remember writing that object, or piece of code.

I've always enjoyed playing games I've made. I had a blast toying around with the editors, making new levels, then leveling up my mage while collecting loot on one of the RPGs I worked on. Sometimes going through stories isn't as exciting when you know the outcome, but you have to learn to live in the moment and just enjoy it for what it is.

On the current project I'm doing now it's not an RPG but a competitive game, and it's been nothing but a great time when testing new builds against the AI.

This is what drives me as a game developer, I actually enjoy making and playing the games I'm working on. This is the type of passion you either have or you don't, and not everyone is like this. I've worked with people who've lost that passion to play their games, but they're very talented and enjoy the creative process. Kinda like building a world, but not actually going down into the world to experience it, and there is nothing wrong with that.