The pill effectiveness- minastrin 24

Can someone please tell me how effective birth control pills actually are? I've been on minastrin 24 fe pills since August, and I take them every day at the exact same time but everytime my boyfriend and I have sex, I get stressed out for the entire rest of the month and freak out until I get my next period. We almost always use condoms too, should I be stressing as much as I am or is it a pointless stress?