Why is it so difficult?

I find that, once I've decided to do something, things get increasingly difficult by the minute. I thought I had lots of ideas, I end up using none of them. I thought my ideas were brilliant, I end up thinking how silly they are. And then there are those who unintentionally put you down with their own negativity. And then there is this brain of mine that can't stop thinking every little thing. So I end up thinking a lot but doing nothing.