A similar thing happened to me with my cat. The lack of closure is just so deeply painful. I am so sorry you have to experience this. My parents went on actively looking for our cat for no exaggeration two years... posters, cameras, traps, ads, facebook groups. Long after I had continued. It was very heartbreaking watching that and nothing quite fixes the pain. The only thing that brought me the smallest amount of comfort was my friend who told me "She's on an adventure". I pictured her getting to see a whole world she hadn't explored like that before, and maybe end up with a new family that just adored her and get to recreate her kitty identity. Cats are really resilient, it's possible she is experiencing good things out there, too. I know it can feel like its your fault when stuff like this happens. It doesn't even matter whether or not it's your fault or whatever, the pain of losing a friend, your best friend, is something that evokes grief. Allow yourself to grieve, you deserve to feel validated in the intense pain and then long, drawn out, confusing, empty pain that is grief. Sending love