There's little I can change about my beliefs that would improve my mood, aside from becoming implausibly optimistic about my future.

How do you think you know that? Maybe some of your beliefs or aliefs are causing wrong actions that are making you sad. From what you say elsewhere in your comment, it sounds like your depression is triggered by romantic failure, so changes to beliefs that help you relate to people better probably could improve your mood. In fact, your particular case of wanting "a relationship . . . in which nobody's deceiving anybody" sounds like a good one for CBT. (Or rather for fixing with rationality-type changes in general, I don't know enough about CBT vs. other therapies to really say.)