I don't think your all gloomy, and I don't think you are thinking too much. You are thinking about the right amount for the situation you are in.
You have a reasonable idea of the timescales to expect from the rest of your life, and the speed cancer can progress.
I think the problem you have at the moment is, you haven't got a firm diagnosis, and based on past experience it will be a number of years before you have one.
Do you really want to be on the treadmill of tests looking for an illusive cancer, which will certainly be found one day if they keep looking hard enough (nearly all men your age have cancer, or will get it soon). Once it is found it may lie dormant for years, but they/you will probably want to treat it straight away.
I can give an example from my own experience with a much less serious disease, which I chose to ignore. I have one copy of the gene for hemochromatosis, in theory I should be fine but I have high iron (serum ferritin diagnosed at 450 normal range 30-300) for two years the hospital bled me every six months, the figure bobbed up and down. Then they failed to call me in, probably lost my records (another story). I ignored the problem for ten years, then with a PSA test I asked to check serum ferritin, it was high 500. GP wrote to hospital, who didn't respond, a year later I asked for another test it was 330, slightly above normal. I could have spent the last decade having pointless treatment if it wasn't for the good fortune of an incompetent hospital.
Rather than hedge my advice with caveats, I'll just tell you not to follow the advice of an idiot on the internet and then tell you what to do.
Ignore the disease until you are 80, maybe have annual PSA tests, but ignore the result if it is less than 30 (it's a big prostate, otherwise I'd have said 20). If the PSA gets above 30 or you get to 80 yo decide what to do then. Hopefully you'll get five worry free years, possibly then another five or ten. At anytime in between you might die, but that's the penalty of living a long life.