World Reset

Hey, to all of you Minecraft developers, I have a issue with Minecraft,I play on PC, I looked around the web and didn't find anything so I hope that you will be able to help me, so, recently the new 1.15 version of Minecraft came out so obviously I downloaded it but after loading into my world I realised that OptiFine wasn't working because it still ran on version 1.14, so I went back to version 1.14, but when I loaded back into my world which was loaded in 1.15, and I loaded it again in 1.14, well the entire world was reset, I kept all my items on me currently but all of my builds, pets and all that stuff was reset back to how it was before I started playing, so essentially I lost everything apart from the few things I had on me, my chests with items in them also disappeared, so does anyone know how to fix this? Please help I spent a lifetime in that world.