This is crucial to finishing a map. I thought this video was pretty good for layout creation ->
https://www.youtube.com/watch?v=vUwcX5Hbdng&list=FLdR9SxCXvkZjZfBdpQEUkyA&index=1
Granted it's CS:GO but as a general principle it works for level design, You can skip to 08:00. The hurdle, I feel, is fleshing out a full layout in the editor and refining it. This is time consuming and at least for me it's not a lot of fun. Your map also looks like crap and it's hard to imagine how this blocky layout is going to look good eventually :P
Here is a video on this process, the making of the CS:GO map de_piranesi. As you can see the designer spends a significant amount of time refining his blocks and playable spaces before adding art!
https://www.youtube.com/watch?v=--FxLX1RhuE
It's not easy but if you can persevere through the layout stages without getting caught up in too much detailing, you stand a good chance of completing a map.