Giving up

I've been trying since march I'm only 17 why is it so hard to get pregnant ? I don't understand all these women and girls have sex once and they get pregnant when they don't even want to. And the people that want to take ages how is that fair ? Every month getting disappointed, now this month even worse I get BFP 3 days after my missed period and now 5 days late 4 BFN and it has just stressed me out and put my hopes up and still no AF not even a sign of AF no cramps absolutely NOTHING!?