"My best friend since kindergarten is bisexual and she has a crush on me but I'm straight and have a boyfriend. What do I do?"
This sounds pretty complicated. Basically, it is really normal for people to have crushes on their friends, even if they have known each other forever. It is also normal to have a crush on someone of the same sex and on someone who is already in a relationship. But just because it is normal for a friend to feel like this, doesn't mean it won't be awkward.
So how should you react? The most important thing is to treat your friend with respect when you explain that you don't return her feelings. Revealing a crush can be really nerve wracking and it is likely that your friend was worried about your reaction. Even though she knew you had a boyfriend, she probably was hoping that you had romantic feelings for her as well.
You might want to talk about what this means for your friendship. Ask her how she wants things to go between the two of you from here on in. Lot's of people are able to remain friends with someone they have a crush on. But it is also common for people to need a little space from a friend who doesn't return their feelings. If that is the case, make sure to give your friend the room she needs. For example, your friend might not want to hear about your relationship with your boyfriend for a while. On the other hand, if you suddenly stop confiding in her, she might feel cut off. Really, the best way to figure out how you should move forward is to be honest, even if the conversation is tricky.
Of course, you also have to decide how you feel about remaining friends. Are you still comfortable spending time with someone you know has a crush on you? If not, you might want to ask yourself what makes you uncomfortable and instead of ending the friendship you could consider addressing your concerns head on. Still not all friendships last forever and that is okay, too.
Navigating the times when a friend has a crush on you or vice versa can be complicated, but at some point or other most people have to deal with this kind of situation, and how you react can make a big difference for everyone involved.
One last thing to keep in mind is to be sure your friend actually feels like this. Now if she told you how she feels directly, then you can be pretty certain of her feelings. But if not, you might want to ask yourself if you are making assumptions or reading into the situation because of her sexual orientation and not because of her actions.