22 & confused

I'm just looking for advice from someone who may of been in a similar situation. I'm 22 TTC baby #3 I've been off birth control since October 2014 my periods have been 4 weeks apart then 8 then 6 their all over the place but now there mostly 35 day long cycles I've tried opk's before but they were never as dark as they should be so I don't know when I ovulate. AF due 10th Aug. I've started using Concie e plus this cycle to see if it will help. Given that we don't always get to have sex when I think my fertile window is because of work. Any advice from women in a similar situation would be great on ovulation etc.