Mental filtering (above) is when you focus on one negative. Discounting the positive takes that one step farther, not only focusing on the negative but also actively rejecting the positive. It is when you reject positive experiences by telling yourself they either don't count or that they are not genuine. Here are some examples:
You are baking cookies for fun . You make five batches of chocolate chip cookies. The last batch burns because you were distracted by something else. You tell yourself "What a mess. I stink at making cookies." You have totally discounted the fact that you made four good batches of cookies and identify your success rate only by the one batch that burned. The positive has been completely discounted in the face of one negative.
Or you have accepted a new job with a new company and you are understandably a little nervous about this new chapter in your life . Your cousin tells you he's been reading up on that company and that he is really excited for you to be able to work there. He thinks their work is good and their company policies enlightened. And he says that he thinks you will really like working there. You tell yourself, "He's just saying that to make me feel good." You discount the positive observations and you also discount the value of the opinion of your cousin who is honestly expressing what he really thinks.
Discounting the positive not only brings you down but it also can sadly stifle your relationships with the people whose words you discount in your cognitively distorted thinking as well as prevent you from learning truths when they are spoken. Fortunately, when you recognize what you are doing you are on the first step to eliminating this nasty source of depression.