Trauma must be processed from the present. If you get lost in the past and lose touch with the here and now, then you’re not processing the trauma as being over; you’re reliving it.
At the same time, you do have to experience the past to process it. You have to allow all of the feelings, sensations, and thoughts of that moment to pass through your present self so they can be translated into normal memories and released.
It’s tricky to balance the two, but it can be done. Have another person with you if needed. Sometimes it helps to have someone who can say “hey, stop and do some Grounding now” if you’re the sort of person that tries to tough it out thinking they can take it all at once. You probably can’t take it all at once without losing your grip on the present. Go slow.
See also: Noticing, Internal Family Systems, Integration