I just can't seem to get everything I want to say, into the story line. And I've been casting about looking at 'self-help' books on Novel writing. I suspect I might be trying to do too much, say too much. I've come to this conclusion by reading up on "Save the Cat" methodology. That story arc in STC sort of forces me to simplify and say a bit less. Do you use, or have faith in the STC methodology? Why? Why Not?
There are lots of methods out there, pick the one that you like the best. I've read the STC books and they don't really do much for me, mostly because they started out for screenwriters and that's not what I'm doing. You have to limit yourself when you're working in non-text media because time is money and money is limited. Mostly, you learn how to put together a cohesive story by doing it. Write whatever you want to write. Expect to cut a lot of it in revision. You're going to get down to the core story line before you're done.
I've used Saved the Cat before. It's a good starting point to story structure and it gave me things to chew on, and some of the lessons I learned from trying it once have helped me in the long run, like how to tie character development to the plot or narrow in on the core story I'm trying to tell, but I no longer use it. I prefer 4 act structure (It's exactly the same as a three-act structure except the second act is split at the midpoint) which I find less restrictive, and I play fast and loose with that. Save the Cat is a valid way to plot a story, but story arcs are not a one size fits all. You can follow Save the Cat strictly, use it as a starting point and add in more, or use a completely different structure, including making up your own structure. For me, writing is an iterative process. No matter how detailed my outline (and boy, do I love to outline) I will continue to need to make changes and add things once I actually start writing. Once I finish the first draft, I'll keep finding weak spots, places where the things I wanted to say don't come through, and I'll have to go back and weave in another layer. Everyone has their own method, but you'll never know everything your novel has to say until you actually write it, so if that's the only thing holding you back, I would say just go for it. You might be trying to do too much...but you can't know until you actually try it.
The recommendation I usually hear is to write the story first and then worry about putting some structure to it in revisions/edits. Otherwise you can structure it too much and fail to let it live and breathe the way a story needs to.