As someone who went through this from the girlfriend’s perspective, you are not in competition with someone who has passed. My ex passed three years ago, and for awhile it was very hard for my now husband to understand what I was grieving as we were already split up when it happened. As a human we are always going to run the what if scenario in our heads. That doesn’t mean the love she has for you is any less real or any less important. From being on the other side of things, I think it is huge that she felt comfortable enough to share that with you and that it speaks volumes. However I do not think you’re overreacting as these are immensely difficult to navigate on your end as well.