3 year old corn died and I feel terrible

One thing I have learned is that snake keeping is a practice. So much of it is circumstantial and we beginners can have difficulty determining what factors to consider. The experts suggest one thing, but we quickly find that it doesn't work for us and our snake. We experiment, we make mistakes, we correct (quickly), we learn. It's a practice.

To the OP, don't beat yourself up. If you successfully kept Steve for three years, and this onset was really so sudden, I have a hard time believing that you did anything wrong. Keeping Corn Snakes isn't rocket science, and they're pretty hardy little guys. It could have been (and likely was) so many things not related to your care. The only practical thing to do is grieve your loss and move on in your own time.

Best Regards.