SUM of cells based on the value of cells on the right and above

Y3 (Total forecast) = Sum of all actuals for the months that have actuals + Sum of all forecast for the remaining months where there are no actuals yet.

Currently, I am using this formula: Y3

=SUMIF(A2:H2, Z2, A3:H3)+SUMIF(I2:X2, Y2, I3:X3)

It works. But I have to manually update the formula every month when actual costs are in. So I want to set up a formula that dynamically changes the sum when the cell refers to "Actual" is not empty.

I've tried SUM with OFFSET, INDIRECT and ADDRESS. Nothing works for me so far.