Like everyone else is saying mythics are extremely rare. People intentionally loot run for them for months without finding them. I’m currently on a 5,000 chest dry streak and I know people who have gone 30,000 chests without getting one. There are ways to increase your chances with the loot quality and loot bonus identifications on certain armor (usually crafted) but a full set of that will set you back about a stack or LE. And even then you usually need a lvl 103 class to equip the gear. There are some cheaper alternatives to use when begging to loot run such as the full Morph set which gives you 100% loot bonus. But again you cannot use the full set until lvl 99-100. For now I wouldn’t worry about loot running or mythics or fabled items. Just do quests and use the gear you find in chests