I define truth as an accurate and reliable reflection of any object in my mind. Of course, I oversimplify the matter to give a short answer. These truths constitute the model of the world we live in (myself included). We may call it knowledge, although some reflections that influence our understanding of the world are subconscious, innate like the language we speak or the body constitution which affect the way we perceive and interact with objects.
Most of our knowledge is a set of beliefs. Where do we get them? Some receive them from a religious authority; these are not verifiable by any rigorous study. Some beliefs are offered by science, a more reliable, controlled and therefore trustworthy knowledge-generating machine. Art is a very powerful source of truths, not directly verifiable by the audience by any practical means: you just feel it when you experience a masterpiece. Street knowledge comes from people who surround us, and we tend to accept these beliefs because we relate to these people. And finally, there are truths (beliefs, knowledge, understanding) about something we uniquely experience as individuals by doing – doing in a very broad sense: by cooking lentils I learn what is the point when they turn soft (yes, there probably is a scientific research covering this, but it does not matter since I acquired this piece of knowledge simply by doing stuff).
Now I can answer the question “Why truths get discarded by harsh reality?”. The ultimate test for any truth is its capacity to withstand the test of practical reality. Practical is a key word here: I need to apply my truths by doing, by watching other people acting, by pursuing specific objectives with the help of the beliefs (knowledge) acquired from various sources. Consider truths as simply means, tools, maps, concepts, gadgets, etc. used to guide us in life. If you get what you want applying your knowledge models, stick to these beliefs; if you fail (because somebody lied, misled, misrepresented, etc. about how this world is and works), then throw these truths away because they led you to failure. I did many times when I failed and was smart enough to learn from it. Failure is your best teacher, and it is embedded in the “harsh reality”.