לוּלֵא הֶאֱמַנְתִּי לִרְאוֹת בְּטוּב ה' בְּאֶרֶץ חַיִּים
If I had not believed I would see the goodness of Hashem in the land of life…!
David doesn’t really complete his thought. If he hadn’t believed that he would see G-d’s goodness, then what? Rashi fleshes it out for us. The previous verse discusses false witnesses who have it in for David. If not for his faith that he would get to see G-d’s promised goodness in the next world, his enemies would have been able to destroy him.