Why am I wanting to end my relationship with a good guy?
Your situation demonstrates the power of perspective and how your state of mind determines what you want. You think you could go without when you have him and are assured. Then, when you are alone, you question it and want him back. You know the relationship serves some emptiness or lack of self-assurance because when you are with him, his niceness reassures you that you are good. You conclude that you don't want to be here anymore from that good space.
It's a pretty good assessment of how we are doing if we are able to be by ourselves or not. Since you are struggling with being alone, I think you know that you aren't ready for it. It seems as though because you are alone, you lose that reassurance and therefore question yourself and question the relationship, and who knows what you start to think disrupting your life.
Now, it's not good to be in a relationship to "make you better" or to "be completed" that is called codependency, and it usually doesn't go well. When you are with him, you feel solid if it is because you are confident with him that you are healthy. If his presence provides a solid you, now you have an unhealthy dependence on him to make you feel better. You are not defined by the person you are with, and if you feel better with them than without, often that can mean you aren't fully established or confident in yourself. You need to ask yourself who you are, what you like, and what those likes and dislikes, independent of others, say about yourself.
Here's the deal with codependency. You are relying on your partner, whom you find unsatisfying, to reassure you of yourself. However, someone else could always come along who does it better or seems to be what your current man is not. It often happens when someone gets into a relationship without knowing it is unhealthy, only to have enough confidence from the relationship to be with someone else. Then the other person leaves them, and they are a mess.
You have to do what serves you best. What serves you is any situation where you get to discover who you are, probably in those uncomfortable alone times. Who you are, what you want, what you need, and how you define yourself. Maybe be alone for a while if you are questioning whether he is the right guy. You can always come back if you are meant to be together. As long as you stay in the relationship, you never get to know who you are when not with that person. Set boundaries with your current significant other, and learn how to cope when you are alone.