Now let's try hear our song at full speed with the backing track.

The ultimate goal with any song you play is to know the parts so well that you can just relax and listen to the vocals while you play. This requires a lot of muscle memory and being super comfortable with the parts, so you won't get there right away. But as your overall skill level improves and you play the same song many times, you get there little by little.

It's just like riding a bike or driving a car. At first it requires your full and undivided attention, and then with enough practice it becomes something you can do without thinking about it. There is probably already something on the guitar that you can do without thinking much about it, and the goal is to little-by-little get all the other skills to that point as well.