1) You had to have some plot (or situation) to begin the book.
Although that obviously underwent revisions; you now have a beginning, and an end. Of the first character(s) introduced, how do they figure in the end, as the last characters doing/saying anything? Those are the endpoints of your arc. Presumably they went up or down in between, but that is the original straight-line plot. I will talk in percentages of the book since I don't know how long it is; and ACTS generally follow percentages.
Alice goes from one state on Page 1% to another state on Page 100%, with "stuff" happening in-between in a way that affects Alice, motivates Alice, or changes her mind until she is the new Alice we see on page 100%, which is where we leave her (perhaps for now, perhaps forever).
Once you have these endpoints, focus on Alice. Fill in this arc: If you are a "natural" storyteller following your instincts; look around page 15% (midpoint of ACT I); you should see something affect Alice that drives her to the end of ACT I. Look there (page 30%), How much has Alice changed by then? She likely has the central dilemma of her arc. Look around page 50% to 60%: You should be seeing the central turning point of the whole story (often a setback, sometimes a new understanding by the characters). Again, how has Alice changed? Consider page 90% to 95%: You should be heading into the final conflict and wrapping things up.
All this applies to your other characters as well, just in a compressed and offset form. If you introduce Barry on page 5%, you might take him to the end (page 100%), or if his job is done before the finale, say goodbye to him at, say, page 85%. Then the middle 80% is how his arc goes from start to end; so see if that is true. Note these are rough percentages; in movies they will often be quite precise but in a novel you have wiggle room of 5% or so of the total length.
2) Use more chapters, separate your characters better.
Chaptering is an art. At the end of a chapter, any span of time may pass, the viewpoint may change, the location may move to another galaxy.
Although I talk about the Three Act Structure often, there are good arguments that few things are actually in three acts, that is just a reference point. A better definition of an ACT is that the ACT is over when something (seemingly) irrevocable happens. A decision is made. Or an action is taken. or a FACT is learned. Alice realizes Bob is a killer; that is the end of an Act. (It is not entirely irrevocable, she may learn she was conned, etc.) Bob realizes he must kill Charlie or go to prison, that is the end of an Act. Bob tapes a bomb to the underside of Charlie's car, and after some consideration, flips the switch to arm it and walks away. End of Act. Or instead, takes it off and leaves, he can't do it. End of Act. Something (big) has changed. Now, it has not yet changed Bob's knowledge that he risks prison if he doesn't kill Charlie, it means he realizes he can't kill Charlie. This is why I say seemingly irrevocable, it feels irrevocable to the character, but in time maybe Bob thinks of another strategy, one he thought impossible before. Maybe he learns Charlie might be bribed.
By this definition, some movies have 30 acts, some books several dozen.
Try to understand where your Acts (by this definition) begin and end. Take note of the changes. Whatever changed in the Act to end it, those are the (many) turning points of your story, and constitute a kind of action-outline of your story. If you have a hundred Acts, or two hundred, A line or two for each is still much more concise than the current several pages you have for each. Look for those "seemingly (to the character) irrevocable change" points as your mile-markers.
You can even use these as chapters: Readers understand chapters have an element of being self-contained in some sense. You can make the chapter run from just after the end of ACT x to the end of ACT (x+1). Then, even if there are things going on with other characters, make sure this chapter focuses on whichever character(s) are going to be influenced by the (one and only) irrevocable change of that chapter.
3) Add complexity gradually. Get back to your simple plot and simple idea.
Begin with that, and add complexity more gradually. Introduce your characters and what they want or need more gradually. Carry the reader along with you, give them time to learn and understand what is going on, and why. There is nothing wrong with having many chapters, give them a chapter each to focus on Alice, then Bob, then Charlie, then David, then Elaine, then Frankie. Any of the other characters can appear in Frankie's introductory chapter and some should, for continuity's sake. But teach slowly. You can introduce Alice and Bob together, but focus on Alice. Then focus on Bob and introduce Charlie. You can jump to David and Elaine, perhaps Elaine interacts with Alice or learns something from her.
Don't be in a rush. Reader's can handle a complex plot, but not if you throw it all at them all at once. Bring them along, one new loop in the knot at a time.
4) Gain perspective.
In order to gain perspective on your whole story, you need to boil it down to its essence. Not be rewriting, but by breaking it up and summarizing it somehow. This "turning point" method is one way; something seemingly irrevocable happening is your segmentation point. Everyplace that happens, so each segment has one and only one turning point. Now you can summarize that segment by its turning point. What happened?
Another definition could be "a scene of continuous time" for some character(s), an unbroken stretch of time while Alice runs through the park or Bob plants his bomb. The turning points generally seem to coincide with the end of such scenes, but not necessarily; so I prefer the turning point which gives something concrete to describe.
In any case, to figure out how to simplify your plot, you need an overview of it you can see all at once; what is major and what is minor. This post is to help you see ways to arrive at that and understand what is important and what is not.
If you have segments that seem to overlap; that may or may not be a good thing, just cut where the turning points are. If your turning point 67 occurs three sentences after turning point 66, no biggie: You can note that 67 requires incidents in 64,65, and 66. To simplify you might consider unraveling that mess into four distinct turning points; but it isn't always possible. IRL some things happen simultaneously; Alice's turning point with Bob may be inextricable from Bob's turning point with Alice. But when they are separable with some imagination, do it: Simpler is better. So two scenes, one about Alice's turning point, a second about Bob's turning point, is better than one scene where they are jumbled together. For example, Alice may decide she is breaking up with Bob in one scene, so the next scene is just informing Bob and him having his crisis over that, instead of both turning points happening in the same scene.