Different cycle lengths ?

AF showed yesterday, 4 days late. That’s the latest I’ve been since we first started TTC in November, and that was back when I was coming off of the Depo shot. How will I know when I ovulate if my cycle lengths vary so much?? Honestly I suffer from anxiety, so I’m highly stressed most months, and I think that might account for why I ovulate late. I’m thinking about starting to take OPKs next month, but I don’t really know where to start. Are my cycle lengths an indication of a problem ? I’m only 19 years old. I really hate how they go from 28 days to 32 back to 29 then up to 34.... etc ... at this point I feel like the only way I could get pregnant is to BD every day of the week that I’m not on my period, and that isn’t possible for us :(