Never pregnant...

I am so anxious to have a sweet baby and I feel like my body is playing tricks on me! Me and my fiancé have been having unprotected sex for 2 years and have yet to have any luck. He has a son from a previous relationship but I do not have any kids and have never been pregnant before. I have a semi-irregular period but it usually comes every month just at different times during the month. However, almost every month I feel like there is a possibility I am pregnant, I have so many symptoms; period is late, exhausted, cravings, randomly nauseous, breasts hurt, moody etc (mind you I don’t have a very bothersome period and it typically doesn’t cause a lot of these symptoms) but this month in particular I am 28 days late in my period and don’t really feel to many symptoms of anything but took a test that was negative. I don’t know if I’m doing this to myself just wanting to be pregnant so bad?? I don’t know how to fight the disappointment that follows it every month. And most importantly I don’t know how after 2 years I have not been able to conceive?