This used to be the hardest question for me to answer, but now it's very simple: My perseverance. I've always been able to make it through much more than I expected. Heck, I started out that way since I was very medically fragile when I was born! I just think about where I am now and I'm incredibly proud at how far I've come.
What's one of your goals right now? Can be simple or complex, a goal is a goal!