Aggregate Function - Combine Results

I have a query more or less like this:

SELECT field1,
COUNT(x) AS COUNT
FROM my_table
GROUP BY field1;

For field1 I want to get a count, but if field1 is like 'ABC%' then I want to combine all of those.

So if I have the following:
ABC1 | 5
ABC2 | 10
XYZ1 | 3

I want results like this:
ABC | 15
XYZ1 | 3

I've tried using some case statements like

SELECT CASE
WHEN field1 LIKE 'ABC%' THEN
'ABC'
ELSE
field1
END AS field1,
COUNT(x) AS COUNT
FROM my_table
GROUP BY CASE
WHEN field1 LIKE 'ABC%' THEN
'ABC'
ELSE
field1
END;

but this just gives me
ABC | 5
ABC | 10
XYZ1 | 3

How can I combine record 1 and 2 from the last record set example above?