The trouble with people saying 'don't feel down' is that.. although its quite often said, it is a rather silly thing to say. It implies that people are depressed through choice, or that they have some direct control over how one feels - we don't. It is no different from saying 'don't be scared', or 'don't be angry' or 'don't feel pain'.
That said, I'm sure some respond well to positivity, do you know one of those relentlessly positive people who can find some joy out of almost everything? If you can't find joy in anything, then faking it till you make it is a worthwhile strategy: if you have the option of going out rather than staying in, alone with your thoughts, go out and "pretend" to be happy. Not a panacea, but it beats the alternative.
It's your last year of college, which means things will change... even if everything is stressful for this year - you know after that year something will change. Which means your situation, like all situations is in a way temporary. That means distraction is another strategy. I'm sure being depressed is a sign that something is missing, or that we are under too much stress, but what makes it worse is over-analysing it. Rather than becoming more introspective (not a bad thing if it isn't constant) try accepting it. You feel how you feel. You've been happy before, and you'll be happy again. You've been upset before, and you'll be upset again.
If a close friend felt how you do, and told you - what advice would you give them? and what would you do to help? Follow your own advice.