Hey, everyone started somewhere.
I think the main thing here though is that you are caught in the 'box' mentality'.
It happens to alot of people, you look at a blank area in Hammer and the first thing you make is a square. It's hard to envision something complex and detailed.
Then you end up with a box.
Some things that I think will help.
1- pick a shape for a layout. Doesn't have to be complex but something that isn't a box. Even Granary to me is just a big box, sure you go through a few doors, wind around a few hallways, but in the end the outside of it is a big box. I hate that, it's so easy for me to see in game.
Instead start with a U, a T, a figure 8 or an S. Something simple but not a square or rectangle. That'll help you optimize too.
Lay down some walls from the top view to rough out that shape.
Then add big blocks. I want a building here. Big block. Break up that large shape into small areas. run around in game and see if there are long views, short views, corners, hiding spots...
add some ramps and balconies.. Then when you are pretty happy with the areas start turning the blocks into buildings with some rooms, doors, window and halls.
When you are done it WONT look like a cube.
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alot of people say don't release you first map. That might be harsh but sometimes you need to just play around a bit and get used to it. Figure out what you might be doing wrong, then start something new from scratch using what you've learned.