Why so difficult

Why does is it seem like when your not trying to get pregnant it happens the first time you have unprotected sex, but when you are trying it seems impossible to get pregnant? I had a baby almost 9 years ago, after missing one BC pill the first time I slept with her father. I was very young then so I was excited to actually plan a baby and be more mature this time and much more prepared.. but I stop taking birth control all together and have been trying for almost 6 months now and every single month I get my period late, I've even gotten it 8 days late one month so my body makes me think I'm pregnant when I'm not. Does anyone else feel extremely frustrated trying so hard when it was so easy the first time?