why do you think I can't get pregnant? 😕

I'm 23 years old, and I've never been on birth control before. Ever. I've had a few serious relationships where I had sex regularly with no condoms or pull out. I guess I just thought, if it happens it happens, and I felt lucky I "didn't need" to worry about birth control... Well now I've been with my boyfriend over 2 and a half years and I'm really wanting to get pregnant. I don't know why it won't just happen. Every time I start my period lately I get depressed. 😞
I've had a few pap's, and my doctor has never mentioned anything about maybe having indometreiosis (I don't know if they even check for that during pap's?) I do smoke and drink sometimes, not excessively, I know that can affect it. I track my ovulation cycle the best I can, but I do have somewhat of an irregular period every once in a while, sometimes being late/early/long/short and throwing off my tracking. I've never talked to my dr about this, but maybe I should. 
How do they check to see if I can get pregnant? I'm not prepared for getting grilled about my personal life from my doctor to figure out why. What do you think is wrong with me? Lol.. thanks for reading and any advice appreciated😊