On the beta when I try to open any world my game crashes or will open for a few seconds and crash

In the beta when I open Minecraft it sometimes crashes right away or will load and when I open a world it will crash, or I will open the world for a few seconds then it will crash. This has been like this for the past 2 beta updates and I haven't been able to play for a good month. My main world is 300 mb and I thought that was the problem but when I tried to open a smaller world it crashed too.