I was just working with one of my authors on her latest work in progress. She ended up doing what, I do believe, a lot of authors end up doing. She got side-tracked. At several points in the story, the central plot was distracted by these annoying little plot twists. So what happened?
In her case, she had created a "backstory" for one of her characters. This created some motivation for things to happen in the beginning of the story. While this seemed like a great idea at the time, it was unnecessary. However, she did put it in there. Now, when the story got going, it was clear that backstory was not necessary, but this is when the problems started showing up. She remembered that this was an issue so she had to bring it back up again to "continue" with the idea. She also had decided on an ending of the story early on that involved that backstory. Ugh!
The thing is, that level of backstory was not necessary. She was trying so hard to work this in that when we got to reading the main story line, we completely forgot about that backstory piece. This should have told her a lot! If you forget about it, then it probably was not necessary.
So, how did we get it fixed? We went back to the beginning. Instead of building that motivation up, we toned it down. We mentioned something about it but that was it. She had devoted paragraphs on that plot element and we brought it back down to a sentence or two.
We see the same thing when authors want to write a series. They spend a ton of time in that earlier book, "setting things up" for characters in later books. You have to remember, you may want to write a series, but the story you are writing IS NOT their story. Back them off!
It is important to keep "an eye on the prize." Know what your story is about and stick to it. Quit focusing on things that do not matter. Your story will flow better and you will be much happier.