I'm in love with an older woman. I'm a fifteen year old girl. She is turning eighteen. She's graduating in June and going off to college in the fall. I'm only a freshman in high school. But we're in the Drama Club together and she's just totally amazing. And we're both bisexual.

We've been hanging out a lot and we've gotten very close. I finally told my other friend that I liked the girl and she was happy. Said all these things about how we apparently flirt and she thinks the girl likes me back.

The next day, the girl I like text messaged me asking if I our friend had asked me about us. That was it. My friend had told her. So I confessed, telling her that I liked her. I was completely ready for rejection. But, that's not what I got. She told me that she kind of liked me too, but she was worried about graduation and leaving. I didn't know how to feel.

But we haven't talked about it since then. Her friend says she doesn't want to hurt me by leaving. Even she thinks it's crazy. Says the relationship wouldn't work.

I need ways to convince her that we should at least try. Or am I just totally crazy for even thinking it might work? Time and time again I've heard "age is only a number" but... it's important here, isn't it?

I think this situation would be really hard. With her going off to college she is going to be a different stage of like and associating with people her age. College is difficult enough that first year let alone trying to have a relationship with a High School sophomore.
Your years of High School should be fun, engaging with kids your age, not worrying about what your college friend may or may not be doing.
These are two different stages of life.

Talk to her, and ask if she wants more than a friendly relationship with you. If she does then talk to her about at least trying things out even if she does leave. If she doesn't think it wouldn't work out then don't force her or feel bad yourself. You'll find someone new and maybe even better for you. But hopefully things work out between you two.