Travis I make a distinction between depression (as in a clinical physiological imbalance) and life simply kicking the crap out of you. In your case, I think your emotional state is entirely normal for someone going through what you're going through. My point being that I doubt you have a mental illness on top of your cancer.
Something else nobody has mentioned is that having cancer doesn't make you immune to Lyme disease. Since you spend a lot of time outdoors, you probably encounter ticks. If I were you I'd get tested.