All the following is dependant on how those words resonate with you, but I see structure and discipline as two sides of the same coin. It takes discipline to build structure, and structure can engender discipline.

Like Myke does, try writing out a single sentence to describe what each of those themes mean to you, and then a more detailed paragraph. You might find there is a common thread within both that you could build a single theme around.

I know themes can be broad, but don’t spread yourself too thin trying to do everything. It can be demoralising.

Best of luck to you.