What is the difference between the two suffixes -pexy and -plasty?

- pexy means you are tacking a (usually sagging) structure to another structure that doesn't move. You take one structure and "fix it to" another structure (this is called "fixation"). E.g., mammopexy is re-suspending a sagging breast by suspending it higher by taking deep stitches into the breast and pulling it upward and suspending the breast higher up by fixating it on the chest wall muscles and bones (which doesn't sag). Hysteropexy is suspending a uterus.

- plasty means reshaping. E.g., mammoplasty is reshaping a breast, e.g., a large breast can be made smaller. Rhinoplasty is reshaping a nose.

Plastic surgery means "surgery with the goal of changing something's shape or form."

Plastik is the German Word for "sculpture." Plastic surgery is "sculpting" the human form.
-pexy = surgical fixation

-plasty = surgical repair