Now it feels like I'm expected to be performing at a high level right from the start of fellowship and I can't meet expectations despite my best efforts.
I don't think anyone expects a fresh first year fellow to know oncology; the whole point of fellowship is to see patients and learn
I suspect you're being overly hard on yourself. I mean, has anyone actually told you that you are not performing adequately?
It's only ~2-3 months in, so unless your program director has pulled you aside and specifically told you things that you need to work on because you've received repeated negative feedback from multiple attendings, I truly doubt you're a "problem child"
if I end up being the weakest fellow relative to my excellent colleagues despite my best efforts is that acceptable to progress to 2nd year and eventually graduate?
Yes, everyone can pass. Your program is also invested in your success and has no incentive to kick you out
And if you're truly a "problem child," you will hear it in the form of negative feedback and be told you're at risk of failing out (very very uncommon)