As I was running last week I thought about quitting things. To be successful at a high level at anything you have to sacrifice other things. In other words to do something well, you have to quit other things.
I was one of those kids that would say when prodded during some sort of exertion, "I am not a quitter!" Much of that comes from my family. We like to finish what we start. However, as I age and progress at a small number of things I choose to focus on those things so that I can take them as far as seems fit. My running is the prefect example of something that I have decided to excel at that I have quit other things to focus on. I no longer play my trumpet or act and I ride my bicycle sparingly. Those have all be incredible life changing events, but at this point in my life I am spending that time finding out how well I can run.
You will quit things in life. Be careful to consider the opportunity costs involved in quitting something. Are you quitting something because you don't like it or because you like something else more or both? It is worth your time to finish things that you start, but it makes sense to pursue things in the long term that can provide you some success. Defining success can be hard, but it makes the result that much more enjoyable.