So the choice is yours! Either edit formulas every month, or redesign the workbook so it keeps track of your profits/losses, AND makes all needed calculations without editing formulas every month.

It is common approach where user sees Excel as some advanced sort of ledger, where user enters some data, and Excel must do some wonders with them. Often it can't. Instead look at your application as at database, and keep data entry and output separate whenever it makes calculations simpler. You make the life easier for yourself, and for Excel too. And you can have practically unlimited number of different outputs (reports) based on same data.