Adding Up Based On IF Statements - Not Enough Nesting Possible

In column A I have the debt name i.e. the company that owes us money.
In column B I state the amount owed to us
In column C the month by which the income is due. This is selected from a drop down list: M1, M2, M3 etc up to M12
In columns D through O, I have the forecast of what payments will be recieved in months 1-12

In the next cell I want to total payment that will be recieved by the due date. So, if the the cell in Coumn C says M5, then it would just add up the amount forecast in months 1-5, but if the first cell equals M2, then it would add up the forecats for M1 and M2. What formula do I put in here?

e.g. Manually enteres numbers would look like this, but I need a formula for cells in column P

A
B
C
D
E
F
G

[Code] .......

The only way I can think of doing this is with nested IFs but the limit is 7 and I need 12. I have used 12 monhts here for example. In fact on the real version I have 48 months and obviously can;t do 48 nested IFs.