Not sure what you have tried, but here's a couple.
1) Grab an "official" level straight from the DOOM2.wad (or DOOM.wad, etc.)...now add and subtract from it. This way you are already "done".... :) Adding in an outdoor area to MAP02, for example.
2) "Mix n' Match". Again, take the core DOOM2.wad. This time, take two different maps (say, MAP01 and MAP02), and copy-->paste one into the other. Now fix all the errors (duplicte tags and whatnot). Now "join" them together with your own section of hallways, stairs, lifts, rooms, etc.
3) Switch to another game. If you normally do DOOM maps, try Heretic. If you make Hexen maps, try ZDOOM. A change of monsters, textures, items, etc. is all it takes sometimes.
4) Switch to a TOTALLY different game. If you make DOOM2 maps, try making something for Quake2, or Quake, or even Unreal or Half-Life. Whatever you have or can scrounge up (most of the "old" games you can find for around $20 at various video game stores or office supply stores...if you are in a city that has such things). Always a treat. :)
5) Go nuts and search the net for "free 3d game engine". Paruse them, pick one, download it, and start trying to figure it out. If you feel like spending some money, you could always pick up one of the "cheap" commercial game engines out there. (for easy of use, and lots o' power, I highly recommend the A5 engine by Conitec; www.conitec.net/a4info.html ).
6) Write a BG story...or five. Fire up TextPad/NotePad/WordPad/StarOffice/whatever, and just start writing a story for a "megawad" or "total conversion". After you've done one, do another. Then another. Don't worry about them being good (or even possible with a DOOM engine). The important thing is to get the juices flowing!
7) Lastly, do something else. Turn off your computer and dust off your old Dungeons & Dragons books. Read. Go fishing/camping for a day or two. Or whatever you can do where you live. In short, take your mind off of DOOM and computers.
Hope this helps!
Paul L. Ming, a.k.a., The_Hierarch on some other boards.