I failed my initial OPC at a similar point. Looking back (that was 7 years ago now) I was being asked some pretty difficult questions for someone of my experience level. I did fail one or two basics though which was fair cop. A reactive WINDSHEAR procedure on approach for example, flying a go around rather than the drill (due to confusion over the difference between predictive and reactive WINDSHEAR at the time). In subsequent SIMs I have seen examiners simply brief and retest, resolving the issue. However this guy was so busy asking me the distance the red CL lights start before the end of the runway and obscure low vis questions we ran out of time.
The most important thing after a SIM which knocks your confidence is to get back on the horse and try to take advice on board. Tech knowledge seems to come easily to some folks. It never has to me, but as with anything it improves over time. If you ask me, expecting deep understanding of systems knowledge from someone with under a years experience on the ac is poor training- unless they were impressed with your standard and were simply digging to see how much you really knew. If that’s the case, you’ve got nothing to worry about.