Does your mother apply the same standard to minors who identify as straight?

I’ve actually met some people who will honestly say, if questioned, that it’s also too young to know for sure if you’re straight, so even though it doesn’t respect self-knowledge it’s consistently applied and therefore doesn’t really read as homophobic. But in most cases this kind of talk is just homophobic invalidation and bringing up the above question can help a person realize what was wrong about them saying you’re too young to know. The exact wording I used there isn’t offensive if you’re reading the exact words, but it could absolutely be taken as aggressive and hostile, especially if said in a (justifiably) defensive tone, so you might want to reword it.

There’s probably also evidence for your conclusion about your sexuality: experienced attraction to the same gender and lack of attraction to members of the opposite gender. While someone could attempt to explain away the lack of attraction to the opposite gender as “just have not met the right person yet,” they must then also apply that to every straight person and consider they might be bi but haven’t met the right same-gender person. Presence of attraction is a lot harder to explain away. “You’re confused,” list out the symptoms and ask how it’s any different from what [straight person in your lives] feels for [their partner], and how they’re sure that straight person is not confused. At the very least, this should open people up to possibilities other than straight. It’s possible to think you are gay and then find you’re bi (or you were actually gay, but sexuality is fluid for some and you changed) with an extremely heavy preference towards the same gender, that’s been an experience I’ve seen in LGBTQ+ communities in the past, but somehow I doubt that your mother is referring to this possibility when she says you’re too young to know.

This is more meant as stuff to say to people who don’t believe you than for what to say in the initial coming-out process :P

You can try to push past your emotions and experience with a bad coming out and tell someone. Hopefully this would be someone who you can point to prior evidence of being accepting—getting another negative response isn’t conducive to most people being willing to come out again. Or you can wait until your feelings change. Or you can just not do it. As someone else said, no pressure to come out, only do it if you want to, and you already managed to come out to us.