First, peace of mind has nothing to do with most of the events in our lives. Now some events are so extreme they are in a different category, but otherwise life will always have times that are uncertain or downright bad, but peace of mind is a state of being some have and s0me don't. If you don't, the usual suspects for getting it are spiritual life, meditation, passion, purpose, etc. These are the same for everyone. Second, everyone has a phobia or undue anxiety about something. If it's just one thing, and in your case especially one thing that actually is happening, you just have to tough it out until it's resolved or accept the situation. Health anxiety is pretty common, and you apparently have a reason to be concerned. Some people have problems and it doesn't really affect them much. Others become crippled and can't focus on anything else. If you're in the latter category, you can't make the problem less uncertain or make it disappear, but you can work on the anxiety through therapy or something else. Also, what happened to others in your family isn't necessarily going to happen to you. Life doesn't work that way. Probabilities are generalizations and don't apply to any particular situation. Don't know what you can do about your health problem, but you can, again, work on not letting it rule your life. Two separate problems.
Ugh, health anxiety. So real and such a pain! Sometimes though we have things that aren't right that do need investigation to make sure they are nothing. Lots of people are naturally nervous and go to the worst case scenario in that process. But when it causes debilitating fear or avoidance or any other extreme reactions, then it is a problem, for sure. I've had this. Sigh. I really have. I fear a couple of health issues and it causes me to avoid taking care of it. Other people, health anxiety is going to cause them to over test and frequent their doctor's office way too much. If you have had testing done and they revealed that you are healthy, then it is time to work on mental health. They used to call health anxiety hypochondria. I really like this article. It is very clear and gives some things to try to see if they help. https://www.nhs.uk/conditions/health-anxiety/ Start by keeping a journal. Not so much of your health but how often you ask for reassurance or look things up about your health and by tracking it, see if you can reduce its frequency. It also emphasizes distraction or keeping busy with other things. This sure can get me out of a rut. Confronting fears can be helpful as well. It then talks about at what point to talk to your doctor about the anxiety. https://www.psychologytoday.com/us/blog/fighting-fear/201305/the-treatment-health-anxiety That article has more information on overcoming health anxiety with some natural tips.
From my reading, I've found that psychotherapy and CBT therapy are very effective for health related anxiety as well. Can you consider something like this?