I suffer the exact same problem.
Apparently the best way to combat it is to set a deadline, be a in mod team or for a competition.
It’s mappers block, it will pass.
A good writer once told me, “you don’t get inspired to write, you write to get inspired”.
Basically, it means you might look into finding a project and/or mod to which you may lend your skills. It helps the project out, gives you more experience, and forces you to get out of your slump. Worked for me when I had trouble writing music.
I get my motivation, inspiration and such for mapping from many places. Pictures, music, books, movies and other games… they all give me ideas for my maps.
What I do, is find an idea that really sticks, is possible, and something I like. That allows me to blow full steam ahead and not suffer any issues. That, or find a reason to do it that really motivates you. (I created a map just to do hl2mp with my friend. :V best map ever)
I always hit issues. Like limited detail sprite (reducing density looks poo), bad lighting on detail models, RTT shadows looking shit, lightmaps messing up, max grid size, simulated shadows, skybox.
Its like the engine is trying to stop me.