Hi Brigitte. Like you I can very easily find myself overwhelmed by the sheer size and complexity of 'the plot' and the way each character has to interact with it, and each other, and the theme, and the various arcs, and the sub-plots, and the structure.... and on and on. It makes my head spin! 😲
I started with one central character, a theme that was driven by that character, and the germ of an idea for a plot that would (I hoped) illuminate that theme while providing both an external and internal challenge for the character. All the other characters, subplots and the rest of the detail have slowly coalesced over time. It's been a long journey, sometimes joyful and sometimes painful... and the end is still not in sight! There are still days when I feel like Sisyphus pushing his boulder up the hill every day. And I've practically bankrupted myself through buying post-its!
Lots of good advice from the other commenters here. One thing that might be helpful from my own perspective was an insight from a writer's YouTube channel, which I found incredibly useful when 'the plot' became overwhelming. That is to stop thinking about the whole thing in its entirety and break it down into smaller, individual plots running in parallel.
So, in my case, there's a 'mystery' plot - the MC is presented with a puzzle, a mystery 'McGuffin', the secret of which she has to solve, and the readers along with her. There's a 'revenge' plot - the MC needs to find and pay back someone who did her wrong in the past. There's a 'redemption' (or 'rebirth') plot - the MC has to grow and conquer her own flaws and misbeliefs to achieve contentment. And there's a 'romance' plot - the MC and another character have to chart their course from uneasy and reluctant allies to friends and eventually lovers.
Those plots are running alongside each other. They often intersect and affect each other of course. But they can be thought of as separate, smaller plots... and are perhaps therefore less daunting.
What I've done is to plan the structure of each of those plots through the course of the book and work out (roughly) what has to happen at various points to achieve the completion of each of them. So when I get up in the morning and approach the day's writing I'm not overwhelmed at the thought of weaving multiple strands and character threads into a huge complex tapestry. I say to myself: "today I'm going to concentrate on 'this stage' of 'this plot'" - which feels like a much smaller and manageable task. And gives me a nice feeling of progress at the end of the day if I've achieved it!
Hope this helps and good luck!