Time

It's easy to think of time as a line, you can easily go back and remember what happened and think of what other possibilities can do for the multiverse. I always thought it would be great if I could use a time machine to go back to myself before high school started and tell myself all the mistakes I would make and somehow erase the "timeline" where I am not at my full potential. From the hundreds of movies we've seen about time travel, we know that it won't work and result in a worse reality. 

I think time is more of a force that exists, one that affects different things in different ways. Like how a feather and a rock act differently with the same amount of gravity. Some things age faster, and some slower, like the lifespan of a mushroom, compared to that of a tree. Time can also exist at different intensities, we know that time moves slower near more massive objects (Interstellar for those of you who have seen it).

The other thing about time being a force means that you can't undo it. If you throw a ball, you can't un-throw it, whatever happened to the ball and its surroundings is final, and it always will be. This is why I think it is impossible to build a fully functional time machine.

That said, it is especially important to keep in mind that there are no real second chances, only new opportunities to make yourself better. Using as many opportunities as possible is the best thing we can do with the time we are given and it is unrealistic to keep hoping for a fairytale ending unless we put in effort to end up where we want to be.