If you are believing something is true which is actually false, and you are really adamant about it, it's going to take some 'undoing' to reverse this sense of conviction. You've probably built up a lot of "story" that explains, justified, rationalizes and establishes "why" this is true.
The more evidence you have gathered and the more you've "proven" it to yourself and talked yourself and others into it, the more volume of material is going to need to be undone. Those who are really extremely convinced of a lie are heavily "conditioned" by all of this psychological support, which may have come from culture or where you live or the kinds of people around you or your upbringing or experiences you had etc.
You'll need to look at what, in your belief system, 'supports' this truth. Look at all the scaffolding you've erected around it, all the supporting ideas and background information, the opinions of others, sources of authority that you believe in that support this truth, experiences you had which led you to be even more convinced this is true, affirmations that you make about its righteousness, ways you spin stories about it which back it up and regurgitate and reinforce it, how you defend it against other truth which seems to not be in accord with it, etc. All of these things add "weight" to this sense of it being real and valuable and correct.
It's not a problem to be correct. It's okay to be correct when you ARE correct. It's okay to be correct about Holy Spirit's truth which IS correct, and a correction of all else. But when you are correct about something that is an error or mistake or faulty conclusion or something based on dishonesty, it's going to present problems for you. You don't REALLY want to be right about how you need to suffer and die, for the sake of some "truth" that the ego has convinced you of, do you?
So as you pull apart your thought system you'll need to realize you've got all this support built up that defends this belief/truth and that all of that needs to be examined with Holy Spirit's light. It needs to be seen from a vantage point of a greater truth, otherwise you cannot recognize the falseness of it. You need to get outside of it and look in from a wider perspective.
For example, let's say you believe that you're in mortal danger because some body part seems to be failing and all the doctors said it's seriously bad and all your family members are showering you with sympathy and all you coworkers are retelling their own stories of woe and you've got yourself all worked up and terrified. Well, now this means that not only do you believe in sickness and so on but you also believe what your family and neighbors and coworkers are all telling you is the truth as well. And maybe you also "taught yourself" that all the doctors are well-trained and totally know what they're talking about, and that you don't dare question their authority. And maybe you yourself have justified to yourself why you need to be sick because it proves how guilty you are about something. All of these attitudes and opinions support keeping the problem real and true for you. You must be willing to question whether ANY of these supports are true.
What if absolutely everyone involved is telling you a lie, to show you what YOU believe that's a lie, and that you need to change your mind about what YOU think is true, so that they will stop telling you you're sure to die? Where is the empowerment and strength in listening to the voice for death? Where is the empowerment in not taking back responsibility for choosing to believe something that is grossly mistaken? You have to dismantle all of the lies that support the main lie in order for the main lie to be recognized as holding no ground. That it has no support. That it can't be justified. That it doesn't have any proof. And thus that you can't believe it. While you think lies are believable, you are stuck. The ego is literally unbelievable.