Header / Detail mixed

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SQL-Server

SQL-Server-2008

T-SQL

I have a table:

IDX	MyText
1	Header1
2	BodyA
3	BodyB
4	Header2
5	BodyC
6	BodyD
7	BodyE
8	Header3
9	BodyF
10	BodyG
11	BodyH

What I want is:

IDX	MyHeader	MyText
2	Header1		BodyA
3	Header1		BodyB
5	Header2		BodyC
6	Header2		BodyD
7	Header2		BodyE
9	Header3		BodyF
10	Header3		BodyG
11	Header3		BodyH

I have tried:

SELECT IDX, MyText AS MyHeader
INTO #Temp1
FROM MyTable
WHERE LEFT(MyText, 7) = 'History'

SELECT
      P.IDX
    , (SELECT TOP(1) T.IDX, T.MyHeader FROM #Temp1 T WHERE T.IDX < P.IDX ORDER BY T.IDX DESC) AS MyHeader
    , P.MyText
FROM MyTable P
WHERE LEFT(P.MyText, 7) <> 'History'

To which I get the error:
Msg 116, Level 16, State 1, Line 3
Only one expression can be specified in the select list when the subquery is not introduced with EXISTS.

Can someone point me in the correct direction?
Thanks