It may be hard to look at ourselves as critically as we can others,but it seems to be necessary now. It looks like you want something from him that you are afraid to ask for lest you lose him. You don't have to be aware of it to be bothered by it. As difficult as it may be , you will need to figure out what it is in order to put it to rest.
I can only guess, from what you've said, but do you think it might be a commitment issue? Two years seems long enough for a relationship to develop into an engagement or a marriage ...... if it will at all.
Ask yourself if you would be content with the relationship if it went no farther than it is now. If the answer is no.... you may have found your problem.