Irregular periods ttc

Bought preseed today, this is my second month ttc. Tracked with opk and Had sex every day of my fertile window last month and it didn’t do the trick so figured I’d give this stuff a shot. Problem is, my periods are irregular. I get a period every month but sometimes it’s in the beginning of the month and sometimes it’s towards the end. I’d hate to use the preseed when I’m not even in my fertile window and waste it. How on earth do I figure out when my fertile window opens? 😭