You have led a dull life? Maybe this will improve. Women help.
2) It is impossible for me to even imagine non-existence.
Hogwash. You just named a concept you cannot imagine? You're playing dumb and you ain't. I can tell dumb from not-dumb, as I have yonks of experience.
3) I understand that in the world I perceive, people cease to exist to me after they die.
Define "exist to me" . People do not "cease to exist". People die. Their life process stops. The evil that they do lives after them.
4) As a materialist I know my mind will cease to be supported by my physical brain after I die, and hence, my mind will cease to exist as well.
No you don't. You infer it from loads of apparently similar cases. You are probably right to do so. (If not, we both got it wrong).
5) My understanding of 3) and 4) are predicated, however, on my own existence.
As is your understanding generally. (Cogito ergot , bread pudding).
6) Thus 1) and 2) seem to be of much greater weight than 3) and 4).
Well, I can't argue about what seems what to you. Fish and chips seem preferable to caviare and cabbage to me. I think your argument has more holes per square potrzebie than Swiss cheese.
7) Thus, it is rational for me to not believe my existence will end upon my death.