Hey, Ryan,
It actually sounds like your ex is pretty sure of what she feels? I don't see how explaining that there's nothing wrong with being aromantic would encourage her to marry you. As an aromantic person who's had exit romantic relationships several times, I think what prompted me to leave wasn't insecurity about my aromantic identity, but stress from trying to force myself to do things I didn't want to, to meet the needs of my non-aromantic partner.
Your ex said that the only reason she didn't want to marry you was "lack of feeling." Forgive me for being blunt, but what is the purpose of asking more questions here, when she's given you such a clear answer? I don't understand what there is to "work out" between you two--if a person doesn't want to do romantic things with you, because they don't have romantic feelings, shouldn't you just accept how they feel, and stop trying to do romantic things with them? If she doesn't want to marry you, or be in a relationship with you anymore, there's nothing you can do to change that. It's not your fault that she has no feelings for you. It's not that you didn't try hard enough as a partner. Feelings don't follow logic and they're outside our realm of control. But what you can control is how you react to how she feels about you. I know that it's hard to let go of her. But it's going to hurt more if you keep holding onto unrealistic expectations for both of you. And if you're holding onto unrealistic expectations until she shuts you down 100%, then you're just delaying the inevitable.
On being aromantic, there are many posts on this site, where people share their experiences of being aro. Medical journals, not so much, but I don't think that scientific research has been very respectful or understanding of the LGBT+ community to start with. I can talk about my experiences with ending relationships as an aromantic person. Maybe they'll help you gain some insight into how she feels.
It took me a long time to realize I was aromantic. The first relationship I had, was with a good friend of mine, and I really wanted things to work out. But throughout the entire relationship, I felt like I was just going through the motions. It stressed me out whenever she wanted to hold hands, kiss, or make out with me. At first I thought it was just social anxiety, and after breaking up with her, I made a point of getting into more relationships to build confidence.
That was a bad idea. I remember Googling "how to break up with someone" moments after agreeing to be their boyfriend. The entire concept of relationships just felt extremely alienating to me. I knew what a good boyfriend was supposed to do, but I didn't understand why. Yes, you do things with your partner, because you love them, but I never felt anything remotely similar to romantic love during all my past relationships. I felt the stress of an actor onstage, of someone pretending to be a person they are not. I felt the excitement of planning gifts for my partners, of optimizing formulas to make them happy; I was more focused on building those formulas, than I was on building a connection with them. I understood romance theoretically, but not in practice. I eventually stopped trying to force myself into romantic relationships, because I realized I was just doing it for the sake of other people around me. I couldn't muster the courage to say "no" to romantic relationships, until I accepted that I didn't want to be in them. And I bet your ex probably thought the same.
I wish you the best of luck in wrestling with your confusion, and in accepting the conclusion of your relationship. I'm sorry that it took something as major as a proposal to end things between you, but with these things, it's better late than never.