Have you ever heard of "post-game depression"? Even if you haven't, there's a good chance you've suffered from it or at the very least experienced some of the symptoms.
Post-game depression, or PGD, is a feeling you get after finishing a video game you particularly enjoyed and the sadness that accompanies the fact that it's now over. I'm currently going through it after finishing Death Stranding 2: On the Beach, and it's happened to me many times in life.
