how to loop through person's work relationship from current to oldest in fast formula

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I am trying to read a person's work relationship from current to oldest in a fast formula. Following is an example:

Rel1- 9/24/15 to 3/29/18  Terminated 

Rel2- 3/30/18 to 1/12/2020   Terminated

Rel3 1/13/2020 to preset. Active

I am trying to use below

count =1

WHILE (PER_EXT_WORK_REL_DATE_START.EXISTS(count)) LOOP

The problem is it works for some people and for some it fails totally. what is the ideal way to traversing employee's historical work relationships