Don’t try to say you want to be friends while hoping she’ll change her mind. You can only be her friend when you’re ready to be a friend with no ulterior motives.
Why not build a platonic relationship with her and have her introduce you to her friends, become part of her social circle, or go out socializing together? Maybe you really like this girl but she’s always seen you as a friend. Whether that’s talking about how good of a friend you are, telling you she doesn’t want to ruin what you have, or straight-up rejecting your advances.
It’s because you didn’t make a strong enough connection in person when you met her.
Stop spending all your time with her hoping one day she’ll see how amazing you are instead of meeting other girls. Otherwise, you’ll always lie your way to the “friend zone”.
And stop guilt-tripping or flipping out on her when she chooses another guy. Those actions are not going to make her think about what a great catch you are. The best way to be seen as a high-value man is to a high-value man. These aren’t tactics to make your friend crawl back to you.
She may even talk about other guys she’s interested in or hooking up with. Learn how to approach and attract new women in any environment.
Stop trying to convince her logically that she should be with you. Stop doing things for her expecting her to sleep with you in return — that’s manipulative. Be more assertive and don’t keep hiding about your romantic intentions with women.