"But my lovingkindness will I not utterly take from him, Nor suffer my faithfulness to fail." (ASV)
Psalm 89 is a declaration of the Lord's favor on David and his descendants. As we know that Christ was from the line of David and we are all grafted in through Him, this promise is granted to Christians as well.
The Bible holds many promises for us. It doesn't say that life will be easy or we'll all be rich and powerful. Rather, He promises that our treasure will be in Heaven. "Lay not up for yourselves treasures upon the earth, where moth and rust consume, and where thieves break through and steal: but lay up for yourselves treasures in heaven, where neither moth nor rust doth consume, and where thieves do not break through nor steal:" (Matthew 6:19-20 ASV)
The greatest promise that we can claim though is that we will always have God's love. "For thy love is better than wine." (Song of Solomon 1:3b ASV) We can be assured that though we still mess up, though we still commit acts of sinful rebellion against God, He loves us as a good Father would. His peace and His grace never leave us. This truth is repeated in benedictions throughout the new testament. "Now the Lord of peace himself give you peace at all times in all ways. The Lord be with you all." (2 Thessalonians 3:16 ASV)
So wherever you are today, and whatever you might have done, be comforted to know that God has promised to us His love and He will not forsake us.