Little thoughts.

You should try out Trenchbroom and watch Dumptruck_ds's videos, its a lot faster to work with than JACK. Though it doesn't really fully support half life it is still has some support and you can just import the map file to jack, export the jmf as a map and import that to trenchbroom, vice versa.

Whenever I make maps, I just go from one room to the next, not really planning a head too much (even if I have a general idea as to what I'm building) I just stick to one thing and then move on to the next, room to room and box to box. I've gotten used to JACK now but I do remember trenchbroom being better to actually make maps with.

I gave it a go just doing Quake stuff and uhm. I think I'll keep trying for a little bit, seeing as I just started using it. But so far I'm actually finding it really hard and annoying to use. Probably just me being used to Jack though...

I totally understand the first point, feeling like mapping takes a looong time, even to do basic rooms. I often struggle with this as well. In fact, it's something I've been dwelling on more recently, in regards to work and my level design side job. Even when I'm working on a map, which I thoroughly enjoy doing, my focus isn't quite 100% or I find ways to procrastinate.

I've been working with Hammer / JACK for 20+ years and TrenchBroom (for work) for around 2 years and I'm pretty familiar with the ins and outs of the software. Still, things seem to take me much longer than other people in the modding community or working on the same project at work.

I've since reached out to my GP about possible undiagnosed ADHD and I'm awaiting a response from a consultant.

This could be a possible explanation and might even be a factor with your second point: Retaining the information that you're taking in while playing official maps for inspiration.

I am more accepting of blockouts these days since I HAVE to do them for work. I can appreciate that it's difficult to get a feel for the map while working with a blockout since I felt the same way initially and I just couldn't make it work. Now however, I think I will be blocking out all of my future levels. Blockouts mean that you can nail down the basic layout and gameplay for your map, and can make adjustments very quickly.

In the past I would just make areas look as nice as possible and then shoehorn gameplay into them, often having to move around a lot of small details where the gameplay demanded it. So my maps looked nice but played poorly. The one exception to this was Bridge the Gap, which I blocked out until I was happy with the gameplay, then textured and lit. The outcome was... a pretty ugly map that played OK. Then I made Bridge the Gap 2.0 based on the first map and put in more effort detailing. So I think blockouts can be tricky to get to grips with at first, but once you realise their true purpose (super basic gameplay focused template which can be quickly edited and compiled) they really do help.

These two points fit my own experience to a T. I've tried making sketches and blockouts before, but none of that helped me design an environment for a map. The sketches didn't go anywhere because I found it annoying to represent a 3d environment in 2d paper (when that was not the point) and the blockouts did little to get me to think of the finer details - and sometimes I'd just stare at the editor, panning the camera view around without any idea what to change next.

For months now I've been trying to make a small campaign, maybe just a single map in size, that I can release and get feedback on to better grasp HL level design before starting to make full-on mods with assets and code and whatnot. I didn't really have a clear goal other than that, so the map keeps changing and changing - it's still a blockout almost entirely made of grid textures - but it's beginning to settle into a defined shape now. I think what made me stick with this particular blockout is that, for all the others, I was trying to either create an entire environment I could "populate" with gameplay later, or build a level around a single central setpiece; but for this one, I tried to make the map complement the gameplay, and that spurred me on to keep adding to it, despite the fact that even the basic layout was still far from clear. Urby is right, blockouts shine when you want to know if what you're doing is fun.

Just like yourself, I want to make maps that make realistic sense, but I feel like I'm becoming more lenient on that front now. After revisiting Valve's maps and playing a bunch of mods to see what I could learn from them, I started to think the map design of classic HL isn't really much less abstract than good ol' Quake, in the "feeling like a real place" sense. And I kinda like that slightly liminal vibe anyway, so now I'm focusing more on making a place that is fun to traverse and fight in, though the realism is still an important factor that determines what kind of place the map will be. For example, I decided to make this map end with the player riding one of those big diagonal elevators up into the darkness, so to make sense, the map will be a deep underground storage area like the one at the start of the Lambda Core chapter. Building it ended up becoming a bit of a cycle where sometimes the gameplay informs what the map will have, and then what the map has determines what kind gameplay fits best, so it's all starting to converge into a clearer picture now.

About Trenchbroom, I actually learned to use it a bit (but didn't make any finished maps) and I can confirm it really is better for translating that 3d space you see in your head into the screen. It can be faster too, when you get used to it and learn the shortcuts - I recommend Markie's tutorials for these. I never tried to use it for Half-Life though, I'll have to see if exporting to J.A.C.K. works smoothly or not.

Oh, and, about ADHD: I've already asked my mom multiple times if I was ever diagnosed with it as a kid, but can't seem to ever remember what she said. Maybe it's because, by the time she answered, I already stopped paying attention...