Stop whatever you are doing right now. Not literally. Keep reading (it's quite interesting). Stop. Whatever. You are doing that you know you should've quit a long time ago. Maybe it is that "dream" project that you started so long ago that you don't even remember why you were so passionate about.
Sunk cost is about the past. Opportunity cost is about the future.
If you are scared to quit (which is absolutely natural), always think of the opportunity cost. Think of the brighter future, not the scary past, and often times that is enough to give you that extra push to make the right decision.
When you finally succeed, but you don’t have the health or someone to share the success with, the project might prove not be be worth it.
Take 5 minutes at the end of the week and reflect on what you have achieved versus what you had to give up to achieve it so you don’t end up giving up too much without even realizing it. Make this a habit.
You know the dilemma. You've been doing something for awhile, but it doesn't seem to be working. Should you keep going, and push through with it? Or quit and do something else? There's no right answer to this. Sometimes, sticking through will be the right answer.
Everyone has a bad day at work now and then. You may furiously leave your office swearing you'll put your two weeks notice in soon. But how do you know when you should give your job a second chance, or when it's really time to quit? For one, you should always follow your gut.