How common is it for someone to question their sexuality and end up being what they thought they were all along. Someone gay starts questioning but ends up gay not bi or straight. Or a straight person to question and end up straight? That kind of thing
Not sure. That would sound more like OCD talking; knowing the truth but still doubting it. Have you been diagnosed with OCD? It's like knowing you locked the door, yet you'll go back several times just to check. It's pointless since you've checked so many times, yet you'll keep doing it. What makes you think you aren't straight? Or mostly straight for that matter?
As to "how common" it is, well, no one is going to be able to give you statistics. And even if we could, it wouldn't help you figure out what your sexuality is, because you're not a statistic, you're an individual. That said, it happens. Some straight people question, and then realize they're straight. It's called "questioning" and not just "gay."
I don't know if it's common, but I feel like I'm stuck in a constant loop of questioning when I know I'm really gay.