I wonder if something like this would work (don't know much about Jet):
select
oi.product_name,
sum(iif(month(o.order_date) = 1, oi.items_purchased_count, 0)) as JAN,
sum(iif(month(o.order_date) = 2, oi.items_purchased_count, 0)) as FEB,
...
from orders o
inner join order_items oi on o.order_id = oi.order_id
group by oi.product_name