Working Cursor But Takes So Much Time (effeciency Problem) Any Help

I wrote a stored procedure (see below), which within a database create a new column (Ordering)and order rows that have the same OID, DECISION_DATE and give a to column ORDERING value 1 to the highest VOTES_REQUIRED VALUE, VALUE 2 to the second Hignest value and so On.
In Other words I am creating a sort on Index (ORDERING Column) on subsets of the database which have same OID and DECISION_DATE based on the value of VOTES_REQUIRED.

The stored procedure is working and I get the right results, but IT TAKES 3 HOURS 30 MINUTES to go through 38,000 rows. I have to run this stored procedure for a database with 200,000 rows (it may take tens of hours...).
Is there any idea to improve the following code to make it run efficiently.

Thank you in Advance.

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CREATE PROCEDURE SP_ORDERING_TEST AS

declare @oldoid varchar(12)
declare @olddecision_date varchar(75)
declare @oid varchar(12)
declare @decision_date varchar(75)
declare @ordering varchar(1)
declare @ordering_count int
declare @votes_required varchar(12)

set @oldoid = 'space'
set @olddecision_date = 'space'
set @oid = 'space'
set @decision_date = 'space'
set @votes_required='space'
set @Decision_id = 'space'
set @ordering = '0'
set @ordering_count = 0

declare review_test_cursor cursor for
select oid,decision_date,votes_required,ordering,decision _id from MYTABLE

open review_test_cursor
fetch review_test_cursor into @oid,@decision_date,@votes_required,
@ordering,@Decision_id

while (@@fetch_status = 0 )
begin
if @oldoid <> @oid or @olddecision_date <> @decision_date
begin
set @oldoid = @oid
set @olddecision_date = @decision_date
set @ordering_count=0
end
update MYTABLE

set ordering = CAST ((@ordering_count + 1) as VARCHAR)
where decision_id = @Decision_id
set @ordering_count = @ordering_count + 1

fetch review_test_cursor into @oid,@decision_date,@votes_required, @ordering,@Decision_id

end

close review_test_cursor
deallocate review_test_cursor

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