Hi Frederick, sorry you're having difficulty.

Yes, CAD modeling tends to be more finicky than poly modeling. That's generally because CAD modeling is trying to generate geometry that accurately represents shapes and not just making a "looks like it" approximation.

There are some tips here for people coming from a poly modeling background:
https://moi3d.com/forum/index.php?webtag=MOI&msg=4865.2

The main concept is in CAD modeling you want to build a simplified extended shape and then cut that up using booleans.

For your case here that means the initial target that you want to get to is this:



You can use Shell to get this:



Then the largest fillets:



Boolean difference with a cutting line to cut it in half:



2nd largest fillets:





So note that once you have formed the initial solid it's good to try and keep working on it as a solid if possible because you can then use booleans to cut pieces.

- Michael