what is causing my guava fruit to drop before it ripens
It is October and has been a drought year.
Generally speaking, when a tree drops its fruit, it's because it doesn't have the energy to hold it; meaning, some kind of environmental stress or disease is affecting your tree. If you have eliminated bugs or disease (inspect the tree carefully, look for discolorations on leaves and bark, look for spots etc on fruit) look to moisture and nutrition. If it's been a dry year, have you provided supplemental irrigation in sufficient quantity? This article should help you with the basics of guava care: https://www.gardeningknowhow.com/edible/fruits/guava/growing-guava-fruit-trees.htm