Game won't load after playing for a while or when I am standing in a nether portal

I have noticed that my game will not load properly after I have played for any given period of time. Some of the faces of blocks would be invisible and my game would enter a loading screen when I try to open my inventory, chat, or the menu. I can usually correct this by closing the game and opening the app back up. This had happened recently when I was in the nether and I was standing in a portal. When I completely closed the game, relaunched the app, and attempted to open my saved game, the world will not load in at all; none of the GUI nor the blocks are visible. I cannot reenter the portal to get back to the overworld; the purple swirls will appear but the game will not let me back into the overworld. I completely closing the game, completely shutting off my phone, but the world will not generate. This bug has completely corrupted by save.