I think many of us here at Scarleteen can relate to one or both of these experiences - of struggling to articulate the nuances of our sexualities, and of feeling like we are invalid in some way.
I'm glad you came to the boards with this because I think feelings of invalidity can really be helped by reaching out to people who have the same experiences as you (hopefully other non-staff members of the community feel comfortable jumping in!).
My personal takes: I think a lot of people who come to the boards with questions about their sexuality feel a sort of obligation to conform completely to the dictionary definition of their chosen label. However, I believe that there's plenty of wiggle room to explore your feelings as they evolve and as you grow into them. Plenty of queer people differentiate their sexual orientation from their romantic orientation - for example, a person may identify as bisexual/homoromantic, meaning they have romantic feelings for people of the same gender while feeling sexual attraction toward more than one gender. If that feels like a good direction for you to describe the way you feel towards other people, great!! But that's just one way you can approach it. It's up to you what language you use to describe yourself to other people (and to yourself).
With the invalidity thing: ah yes, the Discourse. Not only are bisexual people told that their sexuality is invalid by straight people, we also often receive that same attitude from other queer people. People just can't seem to accept/wrap their minds around the idea that you can have feelings for more than one kind of person. And then WE end up with the baggage
I know it can be a lot to ask, but I hope you can become confident enough to push back against that stigma whenever someone directs it at you -- especially when that someone is you. If a girl you're with dislikes the fact that you're also into guys (and vice versa), that's a red flag that indicates that that person is insecure and/or does not respect you.
I think there's also a very popular mindset that often gets circulated on social media that there is something morally wrong or inherently bad/dangerous about dating men (among women and MLM) that, to me, really exemplifies how social media often encourages us to indulge our worst impulses. Assigning moral value to a person's gender and sexuality is what we want to get AWAY from, right? I've been noticing a kind of backlash trend on my various timelines of people being like, "hot take: dating men is Good. I like boys because they are Hot" as a result.
If you want more homework, this advice column might also be a helpful read (and it has related articles linked at the bottom).
Attracted to women 24/7, but attracted to men in my fantasies--does that make me bisexual?