Streamlining, i.e. the Kitchen Sink Syndrome

Actually the Muse is giving you an epic! Or at least enough for a trilogy. Some stuff will let you know that there's more to write, other stuff is for unseen scenery, what you know enabling you to write what goes on.

Focus on what story you're really telling: the MC is grownup and saves whomever against fearful odds. He's got friends and enemies on a rough parallel course ad they al meet at the end.

A handful of flashbacks are okay, the rest save for a second third, etc book, same world, told by same or alt. MC, extending the world, characters, etc.

Ex; book 2, 'My son would grow up to save the world, but first he had to live long enough to do so.' Mum or Dad's tale sets the pre-conflict to book 1 and ends with son at their graves (end of book 1), a phyrric victory.

Write it all down, then ruthlessly edit.

It may turn out that one of these useless nuggets becomes a major plot point. For those that don't, there is always the delete key.

If I have a lot in my head to get down on paper, I write long. I write everything. I stick it all in, even if it doesn't seem relevant. I stuff the pages chock-full of every little nugget I can mine from the Muse's...um...mine.

Yeah.

Then I cut out the irrelevancies on the edit. Sometimes what seems unnecessary develops into a pretty fine plot thread and you never know until you try it.

I've run up against the same thing in my WIP. I've done three things to help me keep focused:
1. Like you I've written all the ideas down (and created a very detailed outline)
2. I've realized that the story will take two books (there's just too much to try to fit into one). Knowing there will be two books has helped me come up with an interesting concept (a kind of bridge between book one and two) that I think agents/publishers/readers will like. Maybe you have to spread your story out as well.
3. I've tried to focus on one key event/character development for each chapter. I have weaved other plot lines in, but concentrating on one thing at a time has helped build the story's foundation and avoid making me feel overwhelmed by the material (the outline has helped in this). It's almost like telling a story each chapter - takes away the "big picture" pressure.
So far, I think it is working. It has certainly helped me cope with the kinds of issues you are experiencing. Two chapters to go, then my betas will be fed. If their appetites are sated, I will call it a success.
Hope this has been helpful.