Hi I'm 25F and my bf is 25M and i feel like it's the end of the world for me

I'm 25/F and my bf is 25/M and this is our situation. I love him so much. I picture a married life with him every single waking hour, even when I sleep and dream. We're running on our 4th month. But during our 1st month, I made the worst choice in my life and hooked up with a past situationship and that lasted for a week. He found out and we talked about it and he forgave me and we've slowly been rebuilding trust ever since. But today out of nowhere he asked me if I ever slept with that person after we started talking and I had to be honest because I did once and told him the truth. He said it's over now and wants nothing to do with me. That's a valid response from his side. If it was me, there would be no reply. I regret the day I made that choice, I had no self control and let my lust get the best of me. I regret doing that to him and it breaks my heart knowing the pain he is in rn. I just love him so much. I'm willing to do anything to make it work. I choose to change for him and for us. I want to be a better person for him not because I have to but because he deserves so much better. I've never loved someone as much as I love him. And I don't want to give up on us so easily. I've told him all this. But I'm afraid he might not be coming back to me this time. I feel like the world is caving in, I feel worthless, I'm not seeking for any pity here. But i feel my heart physically breaking. I can never undo what I did but I wish on everything that I could. I hate myself for making my warm little sun feel like this, I hate myself for what I've done to us. I'm willing to do anything for my sweet, sweet boy and to make him feel okay. How do i fix everything?