In "Na drini ćuprija", we find the word "ustumaran". Well, I don't find this word... How should I understand it? Hvala!
Now that you mention that dictionary entry you found, I suddenly realised it cannot be other than a compounded version of tumarati, which HJP gives as "ići bez određena cilja; bazati, lutati". So ustumarati/ustumariti is the form indicating that the action has advanced/is advancing.
I just couldn't think straight
Yeah, I think you're rightYes, but the meaning of "tumarati" is slightly different, you don't need to be agitated. When you are "ustumaran" you are nervous, anxious, you can't just wait sitting still, you need to move, pace up and down. You know the feeling. The event you expect, you wait for can also be pleasant.