What should I do when I have fallen out of love with someone I dated for 8 years?

Thank you for asking this question, I can sense from the way you write that this is really hard for you. 

Where I feel I could be there for you in disentangling the whole dilemma is in listening at depth to your whole perspective on this situation in therapy.  Through the process of sharing everything that comes up for you as we talk together, you will start to feel more able to gather together your inner courage and conviction to live as you yourself want to live. I strongly feel that it would be unethical of me to guide you towards what you "should" do in anything, including how to end the relationship you write about in the question. 

Therapy works in a far more helpful way than anybody's advice could because it really puts you in the driving seat and nurtures everything you need within yourself to know yourself better and so to become decisive (as far as i understand from your question, making and carrying out a decision is something that you're struggling with right now) because you yourself begin to feel decisive from the inside out.  The process of being in therapy will also help with navigating and building relationships because it builds self awareness, self knowledge and self acceptance.  The more accepting, caring and trusting you feel towards yourself, the more life enhancing your relationships will have the potential to be in the future, as you'll be able to be true to how you really feel as you live them. 

You mention the word courage, I do honestly believe that that is something that comes through self trust and self acceptance.  Those are both attributes that can develop inside you through sharing at depth over a period of time in therapy. When you say you've fallen out of love with the guy you've been dating for 8 years, I'd be really interested to know more about the relationship, how the love was and how it feels now with him.  I'd also like to know more about you and what's going on for you in your life that might also be affecting how you're feeling when you're with him.   

I hope my answer has some meaning for you, it comes from my heart and my experience!

My best wishes, Lucy