Use current value and subtract from multiple cells

Hey!

So i just received a new request from somebody to apply some improvements into an Excel document.
The column "Current bill" changes value throughout the month according to progress on a project.
When it is time to send a new bill each month that equals to the diffrence between current bill and previously billed, we want to easily put out the amount for each category listed vertically.

I have no previous experience in writing VBA scripts, which means i need to be feeded any information with a teaspoon.
What would be the best code build-up in this case?
Preferrably the buttons "january" to "december" should allow for only one click, until it has been reset by another function.

To be clear: The code for March/Mars does not work, but is simply an experiment on my part, but you might see what i want to do there.
The code for February/Februar is not optimal either, i am thinking it should not be necessary with all those lines.

Feel free to suggest simplifications.

worksheet.JPG

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Works like a charm, Thank you!

It is however quite restricted to any changes, since we will most likely have to add more lines for products in between during the project.

Also, i need to find a way to calculate every single row, excluding the title cells, but that might be best to to in another thread?

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