My question is: how can I improve my posture to help my pelvic prolapse from recurring.

The UK NHS website has some information on exercises to help treat prolapse so you may find it useful to read this. I have included the link below:

https://www.nhs.uk/common-health-questions/womens-health/what-are-pelvic-floor-exercises/ 

Other than this, you may wish to use a lumbar support. This is a small cushion the can be placed at the bottom of the back to provide a little extra cushioning and support for the back. You could use it whilst driving or if you are sitting down for long hours at a time.