Holy crap, that's impressive!
And here I thought MY game was big. Just how long have you been developing this game?
Ok, moving on from that. Here's the things that I've been doing in developing my game (which is pretty much what I do for all of my games).
I've had a bunch of different starting points selectable from the main menu. Basically start from Act 2 and start from Act 3. Which set up the necessary variables. If all of the parts work individually, then presumably they'll work together. (I've also tied their visibility to the debug mode, so I don't even have to remove them when I'm finished with the game.)
Another thing I've been doing is quickly going through the part of the game I've just made (basically speedrunning it) to make sure it's completable. Then playing through it slowly, trying to interact with everything while taking notes. Fixing those problems I encountered. And Then getting a family member to play through it while I watch and take notes (it's nice to have a laptop handy, since it's quicker and easier to type, but you might prefer pen and paper).
Next up I'm planning on doing that exact thing, but to the entire game from start to finish, rather than each individual part. So that's a quick run through, a slow playthrough, then alpha testing. Making notes, and then fixing those problems between each playthrough.
After that I plan to get a friend to playthrough the entire game without my oversight, with them giving me any feedback that they might have at the end. I have to ensure the game is as bugfree as possible for this beta testing phase, to ensure they can complete it. (If they meet a game breaking bug or simply get stuck, it might put them off, and they might never finish it.)
But that's just my process. Naturally it's different strokes for different people. That being said, looking through the other responses, it looks like I'm not alone in this approach to development.
That being said, the bug fixing phase really is the part that kills my enthusiasm. I just detest testing!
Thankfully my brother loves testing. And having that source of feedback, critique, helpfulness, as well as someone that is always excited to see what you're making, is by far the most useful thing you can have when developing a game.
I'm quite lucky in that regard.

And here I thought MY game was big. Just how long have you been developing this game?
Ok, moving on from that. Here's the things that I've been doing in developing my game (which is pretty much what I do for all of my games).
I've had a bunch of different starting points selectable from the main menu. Basically start from Act 2 and start from Act 3. Which set up the necessary variables. If all of the parts work individually, then presumably they'll work together. (I've also tied their visibility to the debug mode, so I don't even have to remove them when I'm finished with the game.)
Another thing I've been doing is quickly going through the part of the game I've just made (basically speedrunning it) to make sure it's completable. Then playing through it slowly, trying to interact with everything while taking notes. Fixing those problems I encountered. And Then getting a family member to play through it while I watch and take notes (it's nice to have a laptop handy, since it's quicker and easier to type, but you might prefer pen and paper).
Next up I'm planning on doing that exact thing, but to the entire game from start to finish, rather than each individual part. So that's a quick run through, a slow playthrough, then alpha testing. Making notes, and then fixing those problems between each playthrough.
After that I plan to get a friend to playthrough the entire game without my oversight, with them giving me any feedback that they might have at the end. I have to ensure the game is as bugfree as possible for this beta testing phase, to ensure they can complete it. (If they meet a game breaking bug or simply get stuck, it might put them off, and they might never finish it.)
But that's just my process. Naturally it's different strokes for different people. That being said, looking through the other responses, it looks like I'm not alone in this approach to development.
That being said, the bug fixing phase really is the part that kills my enthusiasm. I just detest testing!
Thankfully my brother loves testing. And having that source of feedback, critique, helpfulness, as well as someone that is always excited to see what you're making, is by far the most useful thing you can have when developing a game.

I'm quite lucky in that regard.
