First: if you're slightly bi, you're slightly bi. I realize that may freak you out, bi-erasure is a thing and especially a thing for men, but that's completely normal.

That said...

Second: you're her girlfriend, bro. You're her gay bestie that she confides in because she feels comfortable with you in a way that she might not with straight guys. Sure, she believes sexuality is fluid, but she is not going to see you that way.

So a big ridiculous love confession is right out. Especially since she's on the spectrum, that's going to be a big fat emotional wallop that she isn't likely to be able to handle.

Hell, it's a terrible idea for straight guys with crushes to do that with neurotypical girls. Chasing Amy was just a movie. But you two? Good GOD.

At least let her know that you think you might be bi. Tell her that you've been noticing that you're interested in women. If she is interested, it's possible that she might say something. (It's possible, women fall for gay guys all the time.)

But don't do the big love confession. Please.