1) in my opinion, there isn't a difference. think of it this way: who someone is attracted to isn't based on some abstract, set-in-stone "sexuality" box we get sorted into at birth. our "orientation" is a word we pick that describes our experiences. for most people, these words are at at best an approximation. the only difference between a heterosexual man who occasionally finds another man attractive and a bisexual man who is primarily into women is that the first man calls & thinks of himself as straight, while the second man doesn't. it's up to you what words you want to use for yourself.

2) it's very possible for someone's attraction to change over time - I used to want to date boys a lot, but now the thought of that is pretty unpleasant for me. I don't know if it's relevant here, but romantic interest and sexual attraction don't always have to go together, you can be attracted to boys but not want to date them, and there are lots of reasons someone could not want to date guys (all of which are valid). and being attracted to someone - or to a whole gender - doesn't mean you *have* to date them.

3) if the thought of dating someone makes you sad, that's a big warning sign something is wrong, and it's not good to ignore that feeling or hope it goes away.

so what if she "turns straight" when she's older? that doesn't change how she feels *now*. if she wants to date men in the future there's nothing stopping her from doing that..... in the future. it's ok to change. it's totally ok to think one thing and realize you were wrong.

you have feelings for a reason. they are trying to tell you something. it something seems wrong for you, it's probably wrong for you. if it seems right, it's probably right! i wouldn't worry too much about being "in denial": if you want to date a girl, then go ahead and do it. if it turns out you're not actually into her, what's the worst that could happen? you have a bad date? you turn out not attracted to her? that happens all the time.

it doesn't matter what your "real" sexuality is. just do what feels right.