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It is difficult to say from the description.
It could be a fungus or watering issues or even damage from sunlight.

Neem Oil can be used to treat pests or most fungus issues.

Here are some articles to help you.

https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-pests-and-diseases.htm
https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm

It can take 8 to 20 years for a seed planted Avocado to produce fruit.
You may also need a pollinator.
Here are some articles with more information.

https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-tree-growing.htm
http://ucavo.ucr.edu/Flowering/RemarkableFlower.html

Here are some articles that will help you pinpoint the issues.

http://www.avocadosource.com/CAS_Yearbooks/CAS_08_1922/CAS_1922-23_PG_10-13.pdf
https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm
https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-fertilizer-tips.htm

Misting the leaves is not necessary. Your avocado plant is in shock from the changed the growing conditions (in this case--repotting). That's why the plant is dropping its leaves. Avocado plants don't like sudden changes. Generally avocado plants don't like inadequate light, too much water (soggy soil), and soil that is too heavy. Make sure that the soil quality you use is spongy, well-drained, and not too wet. Allow the soil to dry out between waterings. Make sure that the avocado plant is getting sufficient light (at least six hours of sunlight), warmth, and dryness. Resist the urge to water slightly moist soil and do not fertilize. Do nothing more than wait it out for a few months. If you run into another problem with insect pests, then use an insecticidal soap (such as neem oil) very sparingly.

There are many, many possible reasons this could be happening. I would examine the tree/leaves thoroughly for signs of insects. Lace bugs, for example, can cause yellowing leaves and defoliation. Leaves that are overall yellow could signal a nitrogen deficiency while yellow leaves with green veins could signal an iron deficiency. Yellow leaves can also be a sign of under-watering or over-watering. More information on these and other possible scenarios are described in detail in the following articles:

'Pests & Diseases of an Avocado Tree':
https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-pests-and-diseases.htm

Problem Diagnosis for Avocado:
ucce.ucdavis.edu/files/datastore/530-27.pdf

That sounds like blight. Prune the affected areas, remove from current pot and wash it out really well. Flush the soil out with distilled water and re-pot. Hope this helps!