It sounds like you will be fine, still keep an eye on yourself for the next hour or so to see what happens, but by all means I am not saying: go panic.
After that hour I guess sleeping would be okay, you were planning on tripping so obviously you can afford staying awake for at least another hour.
Only take the Ativan if you are really panicking despite a big part of the risk appearing to be averted, IMO. Or, if after that hour really very little at all is happening, take it at your own discretion.
Having extensive drug experience has nothing to do with it. Maybe that would help with being able to wing a weird trip better, but it doesn't make your body immune to interacting pharmaceuticals causing toxic effects.
IMO if you learn from a mistake, then you can forget any feelings of guilt. It's repeatedly making the same mistakes that is shameful.