Home Budget: Average cost of an expense for previous 12 months from a specified date

I am doing a budget and want a formula to look at a date I will enter in Sheet1 cell A1 and use this date to look at data stored in Sheet 2 with Column A having a list down of expenses and Column B across having a Month and Year heading with the cost data entered for each item each month. This data is updated monthly and I want to show the previous 12 month average on sheet 1 for each expense from the date I enter in Sheet1 cell A1. I have attached what I have done so far. Thanks for any help