I am totally depressed. I don't know why I feel so.?

I become totally upset for even silly things. I often get confused about things and find very hard to take decisions. Eg, If I go to a textile shop, I see different variety of shirts and get totally confused. At the end, I choose one but after reaching home, I get upset of choosing that particular one and this makes me feel very bad. Please tell me; what is my problem. What should I do?