So I've gone a week or two certain that I was gay. Completely gay. But suddenly I get doubts again. It's always like that - a month or so of knowing that I'm gay, but then a week of doubting.
I've lived that kind of doubt also for about two years. It's hard. And even shortly after coming out as gay, I had doubts of maybe still being hetero. This all cleared up with time and I realised I really am gay. But in the end, what's important is to be yourself. Hetero, gay or bi. No one is pressuring you to be one or the other. Take your time, think things through. Heck, even experiment (safely) with guys or girls if it can help you know your orientation better. The important thing is to not let the perception, expectations or judgment of others / family influence you in who you are.
I think that's ok, and is probably part of this process. I've also been going through periods of certainty and doubt. It's helped me to pick a label but consider my label to be flexible. Sometimes I'm certain I'm gay, it's like, how could I have missed it all this time? Then other times I worry, what if I'm bi, what if I'm ignoring or pushing aside male attractions, and so on. It's helped me to just accept that I'm *pretty sure* I'm gay, but if I find that's wrong later it really is no big deal. It's also helped for me to come out to some friends who I knew would be accepting. It makes me feel more at ease with myself, and less like I have a huge abstract worry that exists only in my own mind.