the past exists as a probable, potential reality. we view physical reality in a linear sequence so that our brain doesn't short out.
from the spiritual level, we can visit any time probablity thread. i also suspect that we change the past every time we make a major shift in our present belief system. IOW, in order to accept a new idea, we change our past. past data must be changed to support the present.
an aside, how many times have you seen relatives debate what 'really' happened at a family event. they were both there and have opposing memories (mary wore a blue dress, no it was green). yet, each believes the other is crazy. now, when mary is asked, she can say it was red, somehow, both will alter their mental picture of the event and see mary in a red dress. simple, i know. sometimes, the best anserws are the easiest. please consider all the implications, before, getting into bad memories, brain chemistry etc.
all probable pasts and futures exist as potential or probable experiences within infinity. the present is where our consciouness focuses, based on the past and future we hold as currently valid.
this makes sense to me, hope it didn't get lost, in my rambling.
peace,