I would recommend making a very basic layout of your entire map first with dev textures. It took me a while to get used to this because I loved detailing and making things look nice. I would never get anywhere whenever I started my map with detailing, I would lose interest. If you have the entire map layout in front of you, It's a lot easier to stay focused on the map because you'll want to work on it. I like detailing and making the map look nice, it's the whole creation of the map layout that used to get me so much. If this strategy doesn't work, sometimes all it takes is just forcing yourself to work on the map. Also playtesting your map after a few updates has always made me want to work more on the map. Once you see it in game, it'll get you excited and make you think of how great it'll look when it's done. Hope this helped!