Use IF condition inside where clause in MYSQL and group by clause

db fiddle

In the table in above link, the balance_aftermonths holds the remaining balance after each month. For current year, I want to check the remaining balance after each month in current year , which I calculate using :

where MONTH(monthlydates) = MONTH(curdate())

However, for the older dates (for example 2019 in example), the total remaining balance should not be current month but the last month in the contract, something like this:

WHERE YEAR(monthlydates) = YEAR(end_date) AND MONTH(monthlydates) = MONTH(end_date) 

QUESTION: My question is that, how these two can be accommodated in a single query?