Consult is very slow

Hi all

I have a table in y database that have 8,000,000 of rows, when I execut a
query on this table, the answuer is very slow.

I have a index in this table by datem, my table is

 Column  |           Type            | Modifiers
---------+---------------------------+-----------
 nombre | character varying(15)     |
 mensaje | character varying(250)    |
 nombre_env  | character varying(100)    |
 cel_env | character varying(15)     |
 fecha   | date                      |
 hora    | time(0) without time zone |
Indexes: correo_fecha

Myindex is:
Index "correo_fecha"
 Column | Type
--------+------
 fecha  | date
btree

My query is

select EXTRACT(MONTH FROM M.fecha) as ho,count(*) as tot from correo M
where EXTRACT(YEAR FROM M.fecha)='2004' group by EXTRACT(MONTH FROM
M.fecha);

When I executed a EXPLAIN ANALIZE, I hace the following Answer

Aggregate  (cost=122439.31..122558.36 rows=2381 width=4) (actual
time=64626.46..76021.93 rows=11 loops=1)
  ->  Group  (cost=122439.31..122498.84 rows=23809 width=4) (actual
time=63951.10..73332.27 rows=4177209 loops=1)
        ->  Sort  (cost=122439.31..122439.31 rows=23809 width=4) (actual
time=63951.09..67240.94 rows=4177209 loops=1)
              ->  Seq Scan on web_sms m  (cost=0.00..120708.48 rows=23809
width=4) (actual time=0.30..55077.31 rows=4177209 loops=1)
Total runtime: 76069.22 msec

How can I do to acceletate the answer?

Thanks.