None of us remember our life before this earth when we lived with our Father in Heaven as His children. However, just because we do not remember this does not mean it was not real or did not happen. Few people remember much of anything from their lives before the age of two. Many people do not remember anything from before the age of four. This is called infantile amnesia - the loss of memory from our earliest years. There are explanations why we do not remember but for now what is important is that even though we do not remember, those years of our lives actually occurred.
We know they occurred because we are here now - reading these words at some later stage in life. We might have photos or video of our youngest years. We might have heard stories about when we were born or about other things early in life. All of our knowledge about our first year or two of our lives comes from other people. We have to trust them (and photographs or video if we have them) but again, we know that those first years of our lives occurred because we are here now. This is the same as with our premortal existence. We do not remember that time but we know it occurred because we are here now. We have the testimony of latter-day prophets to trust. We have scriptural accounts in Abraham and Job referencing our premortal life. We are experiencing an amnesia now but some day we will remember our life before we came to earth.
While there we saw two plans presented - one by Jesus Christ and one by Lucifer. All of us here on earth now chose to follow God's plan - we rejected Satan's plan and watched as he and his followers were cast out - maybe some of us assisted with his expulsion. Evil at that point was definitely out of fashion.
Then we started coming to earth. It did not take long for Adam and Eve's children to leave the Lord and lust after worldly things. They were no longer in the presence of God, although they had access to His Spirit if they sought it. They were, however, in the presence of Satan and his followers, even if they could not see them (although they probably could sometimes). Even though Satan is on the earth, he has no power over people except as they give him power. Make no mistake, he has great power. He tried to kill Joseph Smith as Joseph knelt to pray in what became known as the Sacred Grove. Satan also wields power indirectly through governments, mobs, gangs, murders, lies, wealth, and armies. He calls himself the prince of this world and he is to an extent. He is given nearly free reign over the earth. Satan is very real with real power. Twice in my life I have met people who had evil spirits within them. I still remember how I felt in their presence - it was not good. I have met and talked with many people who were what we would call strong "anti-Mormons" and none of them were like these two men. These two men were full of evil. Satan is there but he only has strength over us when we give it to him.
Throughout the ages God has given us a way to counteract the influence of Satan. He has given us people who speak with authority and clarity against the wiles and insidiousness of Satan. All are entitled to the influence of the Holy Ghost. All are entitled to discern good from evil but the Lord's way is even grander and broader than that. He calls men who have been given His authority and authorization to act in His name.