Look back on your life, and there are bound to be times you felt so down that you felt there's no way you could have carried on. But you made it anyway.
There might also be times you were so happy, with seemingly everything going your way, that you took things for granted. You forgot about the bad times, and forgot to appreciate the good times.
The point here is, life is basically full of ups and downs for everyone. It is a roller coaster which all of us will have to ride. If life is constantly good, then chances are, we will just take it for granted. Maybe you feel that life is constantly bad, but that cannot be true - you just got to look out for and take note of the good things in life.
So, remember, when things are going well, appreciate it, enjoy it. But be conscious of the fact that things might go worsen any time. Don't let it become fear, because that will hamper your living of the present - just be mindful of the fact that life is unpredictable.
And when you're feeling down, do not allow yourself to get bogged down by it. Remember that you just got to survive that period - you've probably done it before. Know that things will get better someday, maybe soon, and realise that this is going to make you a stronger person. Once you have tasted the bitter, the sweet tastes so much better when it comes around.