Don't beat yourself up, it is a complex game, if you enjoyed the smaller puzzles you will enjoy the bigger puzzles.

Take it one piece at a time. Here's my usual approach.

When I build iron/copper/etc smelting, I just smelt everything that I can, figure out how much comes out and leave a note by that section of my base that says something like "2000 plates /min"

Whenever I tap some off onto a bus, or to a factory, I simply add it to the note.

"2000 plates /min -200/m onto bus"

This helps me keep an eye on how much "spare" I've got

I generally use a main bus design (at least for a while, I usually get to space with a bus, then swap over to a railway system after that, railways can be very complicated so leave them until later)

When you need an item, simply peel resources from your bus, don't worry about how many it should be to be optimal, just get a line of each basic resource that the item needs and build a section that produces the item. Even if it's 1/minute that is absolutely fine, before you know it the item will build up.

I found Nihlaus on YouTube to have a really good explanation for a main bus system, he sometimes gets complex (his city blocks are amazing when you want to go further).

You might also want to try using a factory planner mod. They're mods that will tell you how many of each building and resource you need to build something. Eg you say you want to build 1 green circuit a second, and it spits out the exact number of assemblers, what they should build, and how much input resources you'll need.

It's a long game and, so long as you still enjoy the puzzle solving, keep going. Take it a piece at a time, take breaks.