You're focusing on your failure. You don't really want this girl nor will you be truly happy if things ended up working out you simply want affirmation that you are attractive and wanted. In order to have a fulfilling relationship you should first focus on loving yourself, every aspect of yourself even the bad. Come to terms with who you are as a person, and love how unique and individualistic you are rather than chasing after someone else (be it through chasing after a girl / boy or chasing after someone else's image or hobbies etc). People can sense that sort of over-attachment issues and self-deprecation, even if subliminally. How can you love someone else if you don't love yourself first and foremost?
For starters, let's discuss what "oneitis" actually is. Oneitis is basically when people get too attached too easily. It's only human to believe that "this person could be THE ONE" when in reality you probably know little if outright nothing about said person and its just a superficial attraction to them that you feel. Once you understand that putting your feelings too early into a relationship is what develops to attachment issues, that's when you can start beating "oneitis." You have to realize that the things you put out on the table in a relationship are unique things that the other person loses out on, not compare yourself to some other person in a positive / negative manner. Rather than thinking of yourself as a person who "needs improvement" think of yourself as a shop / business. When you put yourself out there for a relationship, you shouldn't have the mindset that you need to "improve" anything, rather you should feel that the other person missed out on whatever aspects you bring to a relationship. If someone doesn't want you then why should you want them? Knowing your worth and not subjugating yourself to the whims of another person is key to this.
The problem I think you are having is that you are simply doing "stuff" in order to try to forget her when instead you should be doing "stuff" simply because you feel like it and because it is interesting to you. Did you get into juggling / song writing simply because you thought you could impress your special person? That's the wrong way to go about it imo, you should do those things because you truly find them fun and exciting. Getting over "oneitis" is simply realizing that no one person is special, the soulmate theory is simply a myth used by people to placate themselves, and to realize that life is far too short to spend it worrying about someone else's opinion. My suggestion to you is to ghost her, get her out of your life as much as possible, delete your social media if you have to. Start focusing on things you want to improve on, rather than things you think someone else would want you to improve (like working out; do it if you find it relaxing / enjoyable but don't get swole simply because you think you'll have an easier time picking up ladies). For me this was guitar, I started learning how to play because I thought it was truly interesting and I find it relaxing and fun, not so I could be that dude who goes to parties and says "hey ladies check this out!" Girls / Boys love that kind of stuff, they enjoy when you are you and finding out all the interesting things that make you special and unique in a normal manner rather than the flashcard equivalent of you just showing off your special skills date 1. Go out places, have your friends introduce yourself to people, quite simply live life and have fun doing so and the relationships will come to you.