You are dating someone you get along with great. You feel great affection towards him; but despite numerous dates, you do not seem to be able to work up any kind of real passion for him. The situation is highly frustrating for you because you would love to feel differently about him and the emotions are just not happening.
In order to develop feelings of love you need to feel comfortable with the other person and let your guard down, so you can slip into a romantic state. But, if your mind is working on doubts you have about marrying this person, you are going to find it hard to let go and allow yourself to develop loving feelings.
If you are still questioning whether you really like this person, or if you are torturing yourself over something you cannot seem to get out of your head, it is much more difficult to develop those warm emotions. Your doubts are what is causing you not to feel love. And then the fact that you do not feel love is feeding into your doubts. Your heart is pushing you forward (because you really like him), but your mind is pushing you back (because you are still thinking it through).
You, therefore, need to go through the issues you have one by one and see if they really are deal-breakers. If they are not, you have to be willing to let them all go and start to “get in the moment.”
You cannot change the effect, namely not being in love, but you can change the cause, which is holding onto your issues. It may not be easy, but it is doable.