How can I keep my Plums from losing their fruit before they get ripe?

It sounds like something is lacking in the soil? Have you done any type of fertilizing?

This is called fruit drop, and is caused by a number of things. Is your tree a younger tree? If it is younger, it could be producing too much fruit (overbearing) and the survival of the fittest kicks in and fruit will drop. Thin out your tree to prevent this. Does you tree need to be pruned anyway? Removed deal limbs, sucker limbs (those growing straight up) and limbs that cross. The canopy needs sunlight. Shape your tree to look like a wine glass. Another reason, is not enough pollination. The flowers are not getting pollinated. Plant companion plants with, or near, your tree. Be sure to invite pollinators to your garden with flowers bees and other pollinators love. Weather: did you have a cold snap after blooming? That can affect it as well. Be sure to protect you tree from weather with special tree covers; or I just use bed sheets. Sprays: are you spraying any insecticides on your tree, or weed killers near by? Both of these will cause fruit drop and possible damage to your tree. Considering doing a "guild" for your tree. There is a lot of info on this online. You basicallly grow a natural mini "forest" around your tree to help it grow better. There are certain plants they help the soil around the tree and help keep pests away. Herbs are great for plum trees. You can plant a ring of rosemary or lavender; for a shorter plant.

May you should be using fertilizer, Your Tree may not have enough strenght to bear all the fruit. Maybe the soil iold and needs to be renewed

You may have curculio. These pests took both of my young plum trees before I knew what was happening. The dropped fruit will have small wounds on them.

what con I do stop this from happening again?

If you have curculio I have read that you should spread a tarp under the tree and shake the limbs. The worms drop onto the tarp and you can throw them in the trash.