Originally Posted by ArviLaanemets

You make here a typical mistake of many Excel users! You are handling your data as some bunch of reports - like paper reports from 100 years back - instead handling them as a database.

Instead you must have a single of data entry table (or a couple of data entry tables on separate sheets in case you can't put all data into single table) - with columns for year and month, or even better a column with month in format "yyyymm". Instead of current monthly tables you must have a single report sheet (designed like your Sheet2), where you select a month (together with year), and get months data read from entry sheet(s). And another sheet (designed like your Sheet1), where totals of every month are calculated from entry sheet(s).

Probably you need also some table(s) where some constants used in calculations are stored.

As result, you get a workbook with 1-3 data entry sheets, and with a couple of report sheets, instead of 12+ sheets, and this workbook will work for many years without any need for redesign, unless there are some major changes in your data structure.