World Crashes When Ever I Get On

I have super flat world that crashes whenever I get on it. This world is really old and I have tried everything to fix it. I have copyed the world, I switched difficultys I even tried adding mods and texture packs. PLEASE HELP THIS WORLD HAS A LOT OF STUFF ON IT AND IT IS MY OLDEST WORLD AND I REALLY WANT IT BACK. It has been like this for a while now so im now reaching out to you guys for help.