It's easy to latch on to girls and idealize them when they're the first girl in the world to be nice to you while not being totally repulsive themselves, but your oneitis isn't special or different from all kinds of different girls out there. Females are literally just carbon copies of eachother with varying degrees of mannerisms based on what benefits them.
Your idealized girl doesn't exist, and never will.
You probably developed feelings for her because she acted like your relationship with her was going to get somewhere, but it didn't. Now that she sees that you're willing to do whatever for her if it means you get to be her friend
, she will keep you "in the zone" , dangling a carrot in front of you and initiating contact with you whenever you get too distant, and pushing you away when you get too close. This is a game foids love playing, and they use it all the time to keep you "in the zone" forever and getting you to do things for them.
Also, the personality of hers that you have come to be fond of is all just an act to get you to do things for her. Foids don't have personality, they only emulate whatever personality will get you to do their bidding.
A good example of this is when they basically give you a job interview or let you talk about yourself to them. This tells them everything they need to know to manipulate you.
Just accept that she will never love you, and drop her. And next time, don't even develop feelings for some female unless she fucks you. That's it. They won't be able to friendzone you anymore or manipulate you anymore.