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.

Overcome the fear of quitting

If your body is aching, if you friends and family are begging you to stop or at least to slow down, if the results are not there, the fun is not there, but you keep on going, it could be because you’ve become immune to the red flags. You’ve learned to tune out the pain and the negative and concentrate on the end goal.
Take a step back and examine the red flags.

Immune to the red flags

Entrepreneurs are especially guilty of this one. You work and you work and fail to take the time and validate your idea first and see if it truly has a potential. Or maybe you are climbing the corporate ladder, but you’ve reached an impasse. 
It helps to take a step back to view the big picture. Talk to someone outside of your usual social circle who can give you an honest opinion about how realistic you are being about the future.

When there is no future

Once you have reached a certain point in a project, in life, or in any activity for that matter, doing more does not necessarily bring your better results. If you want to boil some water, you need to heat it up to exactly 100° C. Anything past that won’t make the water “more boiled”, it is just a waste of time, heat and energy.

Extra effort without extra results

Either the process is enjoyable, or you feel accomplished when you are finally done or you have reached a milestone. 
On the flip side, there are situations or projects that you dread starting, you hate the process, and when you are finally done, you wish you never did it. If there is no benefit of doing it and you hate everything about it, doing more of it won’t help. 5 times 0 is still 0.

When it is not fun

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.  <a href="http://blog.highperformancelifestyle.net/create-habits/">Make this a habit</a>.

Long-term cost outweighs short-term benefits

Things change and you should change with them. Follow your instincts. You know what’s best for you so don’t let the circumstances dictate your life.

Changed priorities

Everything that you do should have a reason that matters and makes sense to you and you should always be in synch with it.The moment the reason is no longer there, or is no longer valid, it is time to move on.

Going it for the wrong reasons

The sunk cost fallacy is one of the primary reasons you are so afraid to quit anything. It occurs when you tell yourself that you can’t quit because of all the time or money you have already spent. The idea of all of that going to waste is what is keeping you at bay, paralyzed at the idea of quitting.

Why you are afraid of quitting

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.

Extra effort without extra results

Quit Already: 7 Ways To Know When It's Time To Cut Your Losses

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How to Know When It's Time to Quit

How to Know When to Quit | Scott H Young

Quitting as a Productivity Habit