Fertility Awareness Method with an irregular cycle?

So I downloaded this app to help me track my cycles and see when I am ovulating in order to help my fiancé and I prevent pregnancy. The problem is that my cycles are SO irregular! Last month it was 45 days and this month I just randomly started my period on day 29! Some months I skip my period completely! I’ve been menstruating for 11 years so I’m not sure why my cycles are still SO irregular, but I’m wondering how y’all deal with that? Since I downloaded the app I also started tracking my CM, which has given me a semi-decent prediction of when I actually ovulated during this cycle, but it wasn’t until after the fact that I realized I had ovulated because I noticed a sharp decrease in my CM after ovulation. So far downloading this app has really only given me MORE anxiety about getting pregnant because I feel like I never actually know when I am fertile and I’m always guessing. I plan to get the Paraguard IUD as soon as possible (calling monday) to hopefully help regulate my period. I’ve never been on hormonal bc before and I would like to avoid it if possible. I appreciate any advice you have about how to deal with irregular periods and FAM.