Advise on getting pregnant?

Got a question? For the last 8 months I've been trying get pregnant me and my fiance have been together for 2 years I'm 18 and will be 19 this year, and I love kids and we are ready to start a family together. We already live together, and are ready for our future together. My periods are irregular most the time, and hard to tell when I may be ovulating. My period calendars tell me when I am and normally we have sex two three days before I do and sometimes the day I am supposed to be. Unfortunately I am still not pregnant and I get upset but I try not to think and stress myself over it like I had been doing. What am I doing wrong? 😕 any advise ladies? It would be appreciated..?