If I can never be certain that someone is telling me the truth, should I always assume they are lying until proven otherwise?

I feel I am a bit naive and have been much of my life.

I have thought people were my friends when they weren't and I have misplaced trust because of it.

I feel most people cowards, so asking them directly about sensitive subjects will more than likely result in them lying.

I am at a loss, how can I trust people and tell them personal things, if I can never be sure if they are telling me the truth.