Is there a formula that would identify the high and low price each month from a watchlist? I would also like the ability to backtest to see high and low each month by year. Thank you.

If you use a monthly time frame, then the high of the current month is just:

H

With the low being:

L

And the high of the previous month is just:

H1

And the low of the previous month is just:

L1

So all you need to know is the number of months ago in order to get the high and low of that month if you use a monthly time frame for the formula.