If I was faithful in the premortal life, why is it so hard to be so now?

All people who come to earth chose in the premortal life to follow Heavenly Father’s plan and to accept Jesus Christ as the Savior. But that doesn’t mean that obedience and righteousness in this life will be easy. Because Heavenly Father’s plan intends for this life to be a true test of how we use our agency, our memories of premortal life have been removed. Also, as part of His plan, we have received physical bodies, which are subject to appetites, lusts, and temptations we did not know as spirits. This makes us more easily enticed by things that are “carnal, sensual, and devilish” (Moses 5:13), which is why “the natural man is an enemy to God” (Mosiah 3:19).

A person can overcome these challenges only if he or she “yields to the enticings of the Holy Spirit, and putteth off the natural man and becometh a saint through the atonement of Christ the Lord” (Mosiah 3:19). It isn’t easy, but Jesus Christ will strengthen you and transform you as you listen to the Spirit, make good choices, and come unto Him.