You don't have to understand or master every ES6 concepts at once. Take your time, and try to practice those concepts in real projects. That's the only way to gain better experience in anything.
Don't forget, take your time...Or what you can do is, learn one single ES6 concept and do small comprehensive exercise every day, and in just a month or less, you'll have a better understanding, it's not a 100m race... Relax