One of the most debilitating aspects of depression is how it flavours all of life with a negative feeling. This goes for your opinions, your expectations and even the way you thing about yourself. It also makes it very hard to see a happy future and to find simple hope in life. Others may tell you stupid things such as "Just think positively" which is nonsense and simply doesn't work. You won't replace negative thoughts with positive ones, the trick to replace them with more balanced ones.
A good place to start may be in how you think about yourself. It's easy to be super critical but those opinions are coming from your depression and they are not reality. You need to be smarter than your depression and think outside of yourself. Be clever enough to not simply accept what your thoughts are telling you, instead, question them. A useful exercise, even if you only do it for one day, can be to catch yourself with every negative thought you have and write it down. By the end a day you could be quite shocked over how much is there. When you're finished go through those thoughts and judge if they are really valid or not.
Probably the single most powerful thing of all which you can do for yourself is to associate with more positive people. The power of someone who is a go-getter in life is potent and over time can have a real impact on how you see the world, they can show you how to view life in a totally different perspective and prove to you that it is possible to live in a very different way from what you're used to. The people you associate with can pull you down or life you up. The choice is yours.