It definitely can be a very fine line. An offshoot of that is something I deal with a lot -- how much of my affective symptoms of my illness are internalized issues I impose on my environment and how much of it is my environment imposing on me? They definitely have root causes and can play off each other -- but for example, right now I am feeling insecure and unable to keep a job. Is it losing my job and having a hard time getting another that is the main cause of these feelings, or is it vice versa? I know the simple answer seems to be "it's all internalized", but that isn't necessarily true. Let's say legal issues are stacked up against you and are causes of a lot of your issues which in turn affect your mood. Again, lots of overlap and it probably doesn't do any good to split hairs.

I guess my point is -- focus on the how it affects you. Be it sadness or depression... if it's affecting your everyday, it's important to note and to try to find ways to remedy that. For both depression and sadness I have heard of a few tricks that seem to work (more effectively for just sadness, obviously, but it does help take the edge off of depression sometimes too).

1) Eat a few ounces of dark chocolate (releases endorphins and some other neurotransmitting chemicals).
2) Spend at least 15 minutes in the sun, or I light therapy works well too I think.
3) Find time in your day to distract yourself with something you enjoy -- watch cat videos on youtube, go for a walk, do a puzzle, etc.
4) Find a reason to laugh -- anything.

5) Journal -- just let the emotions, identified or not, flow. Written or typed , just let it out.

Some tricks obviously are better short term than others, but I hope I helped a little.