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.
And then I try it, but it doesn't work because I still didn't understand the existing code. Rinse, repeat.
However, just the other week I decided somehow to FIGURE OUT HOW IT ALL WORKS, DAMMIT. I actually kept a glass of ice cubes next to my desk and pressed one to my head if I felt I was drifting, which worked surprisingly well. I realized that this codebase always had a lot of the functionality I had always wanted, hidden away in a particular method. I'd been staring right at it for months and it didn't click in my head.
I felt ashamed.