Your journal's an excellent place to mull over challenges in your writing and in your life, especially if you write journal entries in the form of letters.
Just start an entry "Dear Journal", and as you write, you'll find ideas and insights bubbling up from your subconscious mind.
For example, let's say you're stuck in a current project. Start with "Dear Journal" and tell your journal about being stuck. Write as if you're writing to a person. You can even address the journal entry to someone you know, or in the case of fiction, to one of your characters.