The Psalm 115, Verse 18 reads, va'anachnu n'varech kah, m'atah v'ad olam, hallaluka. This means that those who praised Hashem in the world will praise Him in the spiritual eternal world (Radak). The Bais Avrohom points out the Evil Inclination's method of reminding one of his sins while he/she prays. Thus depressed, he/she is stuck spiritually. This verse encourages people to forget their previous failures and feel good as they realize their praise to G-d will last forever.