Description
(((?:string)+)(?!string).([^\n\r]*?)(field))\Z
Replace With: $1\n$2string$3$4

From Notepad ++
From Notepad++, note that you should be using notpad++ version 6.1 or later as there were problems with regular expressions in an older version that have been solved now.
press the ctrlh to enter the find and replace
mode
Select the Regular Expression option
In the "Find what" field enter (((?:string)+)(?!string).([^\n\r]*?)(field))\Z
in the "Replace with" field enter $1\n$2string$3$4
Click Replace all. Each time you click replace all, a new character will be eaten, and a new line will be inserted.
Example
Regex Demo
https://regex101.com/r/jK6jF7/1
Sample text
stringreplacefield
After Replacement
1st time
stringreplacefield
stringstringeplacefield
2nd time
stringreplacefield
stringstringeplacefield
stringstringstringplacefield
3rd time
stringreplacefield
stringstringeplacefield
stringstringstringplacefield
stringstringstringstringlacefield
4th time
stringreplacefield
stringstringeplacefield
stringstringstringplacefield
stringstringstringstringlacefield
stringstringstringstringstringacefield
5th time
stringreplacefield
stringstringeplacefield
stringstringstringplacefield
stringstringstringstringlacefield
stringstringstringstringstringacefield
stringstringstringstringstringstringcefield
6th time
stringreplacefield
stringstringeplacefield
stringstringstringplacefield
stringstringstringstringlacefield
stringstringstringstringstringacefield
stringstringstringstringstringstringcefield
stringstringstringstringstringstringstringefield
7th time
stringreplacefield
stringstringeplacefield
stringstringstringplacefield
stringstringstringstringlacefield
stringstringstringstringstringacefield
stringstringstringstringstringstringcefield
stringstringstringstringstringstringstringefield
stringstringstringstringstringstringstringstringfield
8th time since there are no more characters between "string" and field, then no more replacements are made
stringreplacefield
stringstringeplacefield
stringstringstringplacefield
stringstringstringstringlacefield
stringstringstringstringstringacefield
stringstringstringstringstringstringcefield
stringstringstringstringstringstringstringefield
stringstringstringstringstringstringstringstringfield
Explanation
NODE EXPLANATION
----------------------------------------------------------------------
( group and capture to \1:
----------------------------------------------------------------------
( group and capture to \2:
----------------------------------------------------------------------
(?: group, but do not capture (1 or more
times (matching the most amount
possible)):
----------------------------------------------------------------------
string 'string'
----------------------------------------------------------------------
)+ end of grouping
----------------------------------------------------------------------
) end of \2
----------------------------------------------------------------------
(?! look ahead to see if there is not:
----------------------------------------------------------------------
string 'string'
----------------------------------------------------------------------
) end of look-ahead
----------------------------------------------------------------------
. any character except \n
----------------------------------------------------------------------
( group and capture to \3:
----------------------------------------------------------------------
[^\n\r]*? any character except: '\n' (newline),
'\r' (carriage return) (0 or more
times (matching the least amount
possible))
----------------------------------------------------------------------
) end of \3
----------------------------------------------------------------------
( group and capture to \4:
----------------------------------------------------------------------
field 'field'
----------------------------------------------------------------------
) end of \4
----------------------------------------------------------------------
) end of \1
----------------------------------------------------------------------
\Z before an optional \n, and the end of the
string
----------------------------------------------------------------------