I would be upset with the ending, but I really had a hard time following along as to what happened and why it happened.

Teenager rushes into a relationship that is "abnormal" to begin with (older/same sex), has second thoughts.

I thought it was a nice, bittersweet story. Although I love fluffy stuff too, most relationships that start <18 are gonna end relatively quickly.

The question really is did she back out because it was overwhelming, or because she wasn't a lesbian or bi in the first place? The story doesn't make the question clear, although the fact that she kept comparing the older girl to a guy ("boyfriend", looks like a guy, husky voice, sounds like a male singer she likes, etc.) suggests the latter.

In a funny way, it's sort of the opposite of Our Future Plans.

I think it's because she is still a child that she doesn't fully understand the concept of "adult love" (damn, that sounded dirty) and she knew couldn't be with sensei because of it. Like she said earlier the reason she was uneasy wasn't because of sensei being a woman but something else.