Is this a parent dying that you think about?
I used to force myself to think about my Mom or Dad dying. I was afraid that if I waited to deal with such an emotional issue until it really happened I would totally fall apart and fall into a deep dark depression, lose a job, ruin my relationship etc.
So I thought that by trying to deal with it ahead of time it might kind of "take the edge off it" enough to where I could cope at least a little.
I don't know enough about you to tell you if you have any mental health issues... Are there other things that you "overthink" about? How are your grades if you're in school? Do you have one good friend at least?
And are there any other things going on with the person you imagine dying? Or their marriage?
Try writing a letter to this person and telling them how you feel about them - your fears and everything. Put it under your mattress and when you find yourself thinking too much about it when you should be able to sleep, take it out and read it again...
Is everything else ok in your world??