Let me start by saying that you are not alone. Mapper's block is a very common problem, and it'll pass on its own.
I have had a great deal of trouble with mapper's block. I have mapped for years but have never once gotten anything plast playable betas, either because I lose motivation and don't know how to continue or I get sidetracked by another project/game in its entirety. It's sad really, as all these failed attempts have given me a ton of experience - I just cannot pool my collective skills together and produce anything. You're lucky by comparison.
All I can really say is to not try and force the ideas. Especially in a game like TF2 where we have limited art assets and there's so many geniuses on this forum, you can get demotivated very easily. Just sleep on it. Get a rough image in your head of a particularly interesting-looking control point design, or any section of a potential map. Build off of that. Lots of great maps existed only as rough layouts or ideas in their authors' heads for months before they started being made.
Just go with the flow. And when you do start drawing a layout, stick to it. Do NOT change anything - because the first playable alpha of your map is just that, an alpha - it's for getting insight on your map's layout, and this is the time for you to play around with different things. Do not worry that your map may not be fun to play, or that the layout may be no good - just stick with it. You have to. I wish I could practice what I preached, but I can't.
Good luck!