Cumulative sum by date

Hi everyone,

I have a probably simple problem, but currently do not find an easy solution.
For reference, please see the attached file.
I have a database with project payments (WS Payments) by project and month. In the WS Extract I would like to automatically get the sum for the project indicated in column B up to the month indicated in row 2. I have manually added a formula, but the point is to have a formula that checks in WS Payments for the row of the respective project and then builds a sum from the first months (column C) until the month in which the respective cell is located.

The original data is way more complex and unstructured.

Any advice is appreciated.