All of the things you fear are unknowns or things that are not happening right now. The future is something to plan for, but with the expectation that things can and do change and plans have to be changed too.
What-if thinking is fantasy. It is making things up because the future is not now, and we don't know what the future will be like.
Things that are out of our control, like death, feel scary too.

Are you getting some help from a therapist for your fears so you can feel better?
Your therapist can talk with you about your fears, your fear of death, your fears that make your mind create frightening scenarios that scare you, and that keep you from enjoying your life as it is right now.

It sounds like you will be getting the medical attention for the polyp so it can be removed when it's the right time to do that. If you haven't seen a GI, gastroenterologist doctor, since going to the ER then that would be your next step, and doing that IS in your control.