You can care very deeply about somebody and still not be right for each other.
You are not responsible for giving him what he wants at the expense of what you want or need.
Loneliness is hard, but there are worse things than being alone.
Some things to think about:
You feel that something is missing (has always been missing?) from this relationship. Is it something you need? Are you hoping for something that's reasonable to request? If you do really need it, is it something he might be able to provide? Have you made it clear what you need that you haven't been getting? (Most men are very bad at hints!)
If the thing that's missing is something you've asked him for before, and he has still not given it to you, is it reasonable to keep hoping he will someday? Or would you be asking for a fundamental change that he can't make? (People are who they are.)
If you have a good imagination, how about this: Imagine what it would be like to be married to him, raise kids together (if you want them), and grow old together. Not some idealized version of him, but the real guy you know.
If you cannot see it working out for the long term, then you would do no kindness to either of you by postponing the inevitable.
