Never been to Millbrook myself, being based over here in Somerset, but I've done enough long haul runs through upstate New York back in the 80s and 90s to know that stretch of country has a certain quality to it. Something heavy in the air, if that makes any sense.
Asylums are interesting to me because the photographic evidence that comes out of them tends to be more consistent than most haunted locations. You see the same types of anomalies turning up independently from people who've never spoken to each other. That's worth paying attention to.
What specifically have you encountered there? Shadow figures, sounds, equipment interference? I'd want to know the conditions - time of year, weather, whether you were alone or in a group. Those details matter a lot when you're trying to work out whats genuine and what's suggestion taking hold of someone's nerves.
Anyone else from this forum made the trip out there? Would be good to get a few accounts together and see if theres any pattern to what people are experiencing.