The difference between knowing and doing

Isn't it strange, how hard it is to make what you believe and know rationally in your head influence how you actually feel?

I have so many reasons to be happy right now, but instead, I tend towards the melancholy.

I know that I have been blessed with so much, but so many times, instead of celebrating those blessings, I want more, more, more. I mope about the things that aren't great.

Something small and insignificant can bring me so low. Something that shouldn't matter makes me have really negative thoughts.

I know what I ought to do. I know that I ought to be happy. But it doesn't work out that way.

Why is that? I do not understand myself.