Falling in love with someone who you know will return your feelings is always painful, but so is being in love with a friend (especially if it's a best friend.)
At this point you might be in a dilemma. You're probably thinking, "I Am in Love with My Straight Female Friend. What Should I Do? Should I Tell Her? I Have Strong Feelings?" There's no right answer, and there's no wrong answer either, but there are things you can consider doing before jumping to a conclusion.
Ask for their advice, whether or not they think you should confront your female friend. It's especially helpful if they're close to you both so that they can help figuring out what's best for both of you.
If it's gotten to the point where she constantly gives you mixed signals and you're sick of being confused, then it's about time you confront her. Hiding your feelings for too long can be bad too, as they might explode at any time. (Also read How to Tell Your Crush You Want to be More Than Friends and 35+ Romantic Things to Say to Your Girl Crush.)
Unlike confessing, confronting can just mean talking to her and letting her know how you feel. This especially applies if she's your best friend, someone you're very close with. If she really is your best friend, chances are she'll understand where you're going and nothing will change, especially your friendship with her. (Also read How to Ask Your Crush Out without Embarrassing Yourself.)
If you were never close with her to begin with, then it's best if you just distanced yourself from her. Find other female friends who you can relate to and will reciprocate your feelings.
Talking in person is always better, but sometimes it's a lot easier to just confess through text. However, if you're not out to her yet, then it's much better to talk to her in person. That way, you can see her true, genuine reaction. She'll also appreciate it much more as it's more intimate and shows that you trust her. (Also read How to Tell Someone You’re Gay Without Saying It.)
If you choose not to directly confess to her or confront her, then you might want to try indirectly asking her about her feelings regarding your friendship. She may feel like she only loves you as a friend, and that's okay, but the more you dig deeper, the more you will be able to figure out her true feelings towards you.
As painful as it is to experience unrequited love, sometimes it's better to let it out rather than bottling everything in. Even if you think the other person will respond negatively, it'll be like lifting a weight off your shoulders once the truth is out.