If you don't want someone to ask, just say that you ARE fine. If you don't want to lie you can say: I don't want to talk about it.

Well, usually asking someone Wie geht's? does not mean that you really want to know details. So a person not being your friend would not ask any more questions and a real friend would probably ask more questions when you tell him that you are not fine. But I guess that would work: 1. Nicht so gut. Ich erzähle es dir später. 2. Nicht so gut. Frag jetzt bitte nicht warum.

Certainly. You could say something like, "I'm really not doing well, but I don't want to talk about it right now." This politely communicates your feelings while also setting a boundary about discussing it further.