I need to create a formula that acts like a combination of a SUMIF and a
VLOOKUP. I thought a DSUM would work, but haven't had any luck.

I have 2 sheets. The first contains Account numbers in the first column,
then 12 further columns for each of the months, containing dollar values.
Accounts may appear more than once on this sheet.

The second sheet has a column for the various accounts, each listed once.
Depending on how many months are "Actual" versus "Forecast", it needs to sum
a given column in the first sheet. So if six months are actuals, it would go
six columns over and sum all of the accounts (I have a reference cell setup
to count them and give a number).

A vlookup would be perfect if it could sum multiple accounts in the first
sheet. A sumif would be perfect if I could change the third term to go over
X number of columns depending on a certain cell. Since neither of these
seems to be possible, any help you could provide would be greatly appreciated.