Firstly, don’t get disheartened if you have been learning piano for a few years and you still can’t sight read very well.
What you are trying to achieve is enormous. You are trying to learn how to read in a completely new language. If you have never thought about music as a different language before, here is a definition of “language” from dictionary.com:
"any system of formalized symbols, signs, sounds, gestures, or the like used or conceived as a means of communicating thought, emotion etc.: the lanugage of mathematics; sign language."
Music is often referred to as the universal language, because the message it conveys transcends barriers of spoken word between people from different countries.