Need help for a automation Formula in Excel with automatic average

I am attaching a sample excel where you can help me.

Explanation:

To keep an eye on our finances, we want to enter the real costs for each past month and for the coming months we want to automatically create an average from these past months to get a "forecast".

Creating the average is easy, but we want that as soon as we enter a real number for the new month (i.e. the forecast status is canceled because real costs have come about), the average value for the coming forecast values is automatically adjusted and the average value changes to 4 months instead of 3 months, for example.

does anyone have an idea for a formula?