Prior balance (Spanning in years and months)

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Hi

I need to do the following scenario, where i need to get the prior month balance + current month balance.

but when it spans over the next year, @prior function does not work. For Apr, even if i write separately another function to get the balance from march, other months become inaccurate as it is a moving balance.

Highlighted in cream color, is what im trying to achieve. periods and years are variable based on the date entered by the user.

Following script does not work. even if i put a LOOP to it. because once apr gets updated, other months have to. and then once other months get updated, apr have to be updated.