When I feel down with some bad news and don't want to talk with anyone, but I have to. I hide my real emotions and act cheerful. In this case, how can I describe myself, "I fooled myself." or "I faked myself." I'm very nervous at the speech in front of the people, but in order to hide my nervousness, I pretend to be confident. In this situation, can I use the either sentence? Or if there are any other sentences for these situations, could you explain them?
Neither of these phrases are appropriate for what you are trying to express.
To fool someone means to deceive them, so saying that you "fool yourself" would mean that you are tricking yourself into believing something that is false.
The phrase "fake myself" sounds odd. It would be better to describe the state that is being falsified, e.g. "fake confidence."
In my opinion, other words are better used to describe this situation, many of which you used in the body of your question. The verb "act" implies that one is faking. To say that you are "hiding" your true emotions is also a valid way to express this. Here are some more suggestions:
I acted strong despite my weakness.
I chose to hide my anxiety.
I smiled with manufactured confidence.
I responded with optimism I did not feel.
I tried to cover up my mood with pretend cheerfulness.
(Note that "pretend" is used as an adjective in my last example.)
Another word you might consider is "feign", as in "to feign confidence".
You put on a brave face when you don't want anyone to know you are scared or timid.
Put on a happy face when you're sad or feeling down.
I'm not entirely convinced with your choice of expressions. To fool someone / yourself means you are misleading or lying to either yourself or another person.
You can't fake yourself that is impossible, but you can be called (a) fake, a person who pretends to be something he or she isn't. So you could say about yourself: "I'm (a) fake" or "I faked it" i.e happiness, confidence etc... but both are very negative and don't show that you are/were trying to overcome your sadness or anxiety.