If we remain passionate about learning new skills and preparing our mind for our intended path, we can do many different things and find success in all of our endeavors. However, like you mentioned, must keep in mind the destination and the value of the achievements we're after. Some are going to pay far greater returns than others.
I also believe that failing and learning harsh lessons can break a person in a way that either guarantees self-destruction OR success. A person that knows what it is to fail or have their dreams crushed is better equipped for success, in my opinion. Many great contributors to our world truly failed their way to success.
Yeah, I was forced to quit airborne school because of illness. I hate quitting. I still wish I could go back and fix that situation after all these years too. It still bothers me. The book "Grit" is awesome. If you have the time to get and read it, I highly recommend it. One of my greatest strengths is grit. I just refuse to give up when I've decided to do something. Part of that is being forced to quit in the past too.