I do some mapping as well, and as I suffer from "Play or Map?" (Cake or Death?) syndrome as well, I tend to map a bit differently than others.
1) I make a grand overview of the map. (This specific map is a Jail CSS map, so I have a Counterterrorist spawn room, a cafeteria, main jail cell, holding cell, pool, field, towers, etc, etc).
2) Then I decide which bit would be fun to do first. (I picked Counterterrorist spawn room)
3) Then I draw a few detailed sketches of that room.
4) Then I start building that room.
5) I use the cordion tool (It's the tool along the top with the black and yellow striped border) to build a cordion around my map, as it exists.
6) Then I playtest my "map." This is to encourage me that I'm actually building something that will work. There's not much there, but there is PROGRESS!
7) Go back to 4, 3, or 2, depending on if the room is finished or not.
This constantly encourages me to keep working on the map, because I get evidence that there's something being built, other than boxes in Hammer.
Also, do a little bit of mapping every day. Mapping isn't hard or stressful, so putting an extra room, for a few detail entities on a map, keeps it going forward, so the next time you sit down, there's a little bit more done.
Release Early, Release Often. The Open Source mantra works with mapping too. You're not Valve, you don't have to keep your maps secret. Get the map roughed out. Don't spend too much time with details, just get it working so people can play on it. This way you can get comments telling you that the sniper tower doesn't have a clear line of sight to the front door like it should BEFORE you build that elaborate Greek Statue tower in the spot you need cleared out. You'll also get encouragement from the community about your map. And maybe some ideas.