Definitely familiar. What I would suggest is to figure out what it is you're really getting hung up on. It has to be something more specific than "my brain turns to mush." Try writing down a description of the problem you think you're having. The act of rendering a thought into writing will often break a logjam for me.
I get hung up in a number of ways, but the most severe is when I'm trying to learn how the existing system works. I'll be listlessly paging back and forth between spaghetti code classes, and things get all fuzzy, and then it feels like I've spent enough time on this, and some sort of strategy to do my project occurs to me. And then I try it, but it doesn't work because I still didn't understand the existing code. Rin…
Personally I would only try to understand the parts I currently need.
Yes, in theory it would be nice to understand everything before you start. It would also be nice to understand quantum physics, and all sorts of other things. In real life, though, sometimes you have to compromise.
I've had the same experience with specifications, btw. Yes, I'll read the whole document before starting. But usually I only really understand it once I start implementing the details. (I am also not convinced that specs of several hundred pages are any use at all).