I know that feel bro. Lord almighty do I know that feel. I have combated it since day 1. I'll shamelessly point you to a few posts from my blog
. (the first two most relevantly).
I am on the complete opposite side of marnamai. Don't plan anything out. Don't create in terms of a whole map. Your attention span is short at the moment. You have fantastical ideas of awesome creations when you lay awake in bed at night but in the morning when you go to make them happen they just don't hold your attention. So work in terms of sessions. Make one wall and make it awesome. Throw it out at the end of that session. It's easier to do this with models since they take much less time than a whole map. I find that when I can't keep focus it's because I don't have any immediate feedback. So provide yourself with some immediate feedback. I much prefer to detail than block out (and I know it's a bad habit but fuck you it's the only way I can get shit done). So I often detail as much as I want, and then block out for an equal amount of time.
I find that I work far better when I am learning something new, or reinforcing stuff I already know. So try to work in terms of education. Just because you know how to place a light in hammer doesn't mean you know how to get the most out of that light. Study other maps, take a random tutorial (you'd be surprised how much you can learn from someone who works in a different way) emulate the early days when you were learning and mapping like a machine.
Also I have a really bad habit of losing all motivation once I have shown a project to anyone. Once I show a WIP and get feedback, it's like my desire for praise is sated, and I no longer need to work anymore. It's another stupid thing my brain does to fuck me over. I never announce anything I am doing anymore until I consider it done. At most I will ask for opinions on small areas that I think will benefit from input. Also once I show a WIP I am afraid to destroy the good progress I have made so far, so I stop work altogether. Don't be afraid to fuck it all up. If you are really scared, save a copy of your project before you embark on the next stage. You can always come back to it. Fuck ups are how we learn, and they are a completely essential part of the process of making anything that's worth making. You have to embrace the fuck ups.
Which leads me to my final point, everything takes time. I used to get really down on myself for spending days (yes, DAYS) on the flow of a single loop around a model, or the lighting in a single room. You need to embrace the fact that your project is going to take copious amounts of time, but it's not time wasted, it's research, it's education, it's doing what you want to do. What will you do otherwise? Watch TV? A project takes a set amount of time. Whether you do three hours now or three hours later, it will still take three hours. It's up to you to decide when those three hours end. Today or next week?
One last aside, avoid burnouts. They are the bane of the modders existence. Take breaks. Take vacations. You'll find time and time again you eventually come back to it without meaning to. That's what makes the difference between a chore and a hobby.
I first touched hammer sometime around 2004. I have never released a map, or been on a team that has released a map.