Psychology had to be a science studying behaviour and mind of living beings.
There is a separate part of it studying specifically human behaviour and mind.
However, there is absolutely no any definition of how exactly human behaviour and mind different from non-human ones and how to separate one from another.
So, psychology can't even give an answer to the very basic question that obviously should precede any further study - is some person mind and behaviour human or non-human?
Psychology could have been very useful instrument to detect those with non-human behaviour and mind among us with scientific precision, but it was purposedly turned into pseudoscience bullshit to not even allow that question.
What do you mean by "we forgot it"? The historical accounts got destroyed? I don't see a good reason to believe in that. I mean some knowledge inevitably got destroyed and lost but I don't think knowledge is the problem but rather its interpretation and evaluation which is done by people and institutions. We supposedly have more knowledge now than any time in known history and yet look how things are going. Man's problem is moral, not rational (by moral I mean having knowledge of the truth and the good and still rejecting it for falsehood and evil; or constantly choosing and worshipping death and nihilism over life and purpose).
Christianity has all the answers on the human condition - man was created in God's image (perfectly good) but fell away from God because he chose to follow Satan who seduced him with lies that man can become "like god" himself if he defied his loving creator. That's how evil and death entered the world and the only way to mend this, to return to Eden and gain back eternal life is to get reunited with God (theosis), because God is the source of the uncreated light, truth, the good and life.
As you can see, all this knowledge is safely kept within the Church and remains unchanged for thousands of years. People have known this and even if they are not well versed in Christian theology have that knowledge in their hearts. And still people jump through hoops not to believe in it, mostly out of pride and rebelliousness (just like Adam). That's how one gets deceived to act against their own interest and against their creator. I was exactly like that too when I was younger.
Exactly, that's why a materialist approach to psychology doesn't work. You can't appeal to nature (hardware) to show you what the software (mind, soul) is. This is most evident in the field of ethics - looking at nature can't tell you what's right or wrong - it just is. Only a evaluation by a mind that has conception of morality can do that and concepts are not found in the material world or subject to empirical observation.