Minecraft Worlds Not Loading

Whenever I create a new world I just fall, the world doesn't seem to load and I cant type in any commands. It did work a few times however Mobs wouldn't move and I couldn't interact with anything due to it not loading right (I'm guessing) If I want to play I have to download a map, However sometimes that doesn't work and just lags a lot. Or does what I said before about falling endlessly. Going to the end is the same (Not tried nether) However it will load very slowly but mobs wont move and I cant interact. Forcing me to restart the world.