World Glitch

I was making a perfect world because I was making a really good city and for some reason this glitch popped up it's like im in a flat world but then infinite world adds in with it like the flat part is surrounded by it and I'm mad because I spent all my time on it and I put a lot of effort in it and i never wanted it to mess up
P.s I always will like Minecraft and I will always play it but it's just the dumb glitch that's making so frustrated cuz I want it out my world so I can finish my nice city and I got pics of the city and the glitch and when I go to the world it says I'm in survival but then when I join it doesn't show the hearts or anything then it adds zombies and spawns them in different places and it turns off cheats