Everyone, at some point, feels a little down. It is simply a part of the human condition. We should stop telling ourselves that it's somehow bad to have these moments. Physical pain is a way of your body telling you that something is wrong. Like that, these moments are pregnant with information that reveal what is wrong from another, and deeper, level.
Now, what I am not saying is that we need to feel sadness in order to feel happiness. No.
What I am saying though is when or if you do feel down do not be so quick to move from that place as it is the source of great insight into the human condition - into your personal condition. We are all different in this regard. Only you can know what your condition is like. Only you can see it. Therefore only you can fix it. Others can help but in the end only you can fix yourself.
So, next time you feel down do not distract ourself with some meaningless activity. Don't just go and do something, anything, in order to make yourself feel better, or at least not feel bad. Instead use that time wisely. Spend some time in that place. Sitting quietly watching the feelings. It might reveal something. But be gentle while there for it's a fragile space and one must be careful. Look too closely and the feeling will dissolve. Don't look at all and knowledge will not reveal itself.
Overtime and with practice I promise it becomes easier.