you have to understand that you can love someone without being with them
people with borderline personality disorder (i dont think you have that) have a problem with not being able to understand that other people exist independently of them. they think that other people are just extensions of them instead of seperate individuals, which is why they get so possessive - they think other people in the world basically belong to them and are disobeying them when they see them doing what they want to do instead of what the borderline wants them to do. borderlines never really fall in love as a result because instead of loving a person as they are they just use them as ways to help create a better self image of themselves. for example a borderline wont love you because of you, they'll "love" you because you help them feel like theyre a good person, or sexy, or powerful, or smart etc. they basically just use people like toys to play an imaginary game where they are the starring role
i use that example not because i think thats what you are but because when i had to learn about borderlines and how terrible they are it helped me understand that i never want to be like them. you DONT have to be with someone to love them. if you love someone, understand that they dont have to reciprocate that feeling back to you for it to be ok to love them anyway. it doesnt matter what that person thinks about you (if they laugh at your jokes, tell you how great, or how smart you are etc) for you to have an ability to love them. you DONT have to own someone to love them. love at its core is a selfless act. it has nothing to do with you and everything to do with a selfless appreciation of the other person. if you really love that girl like you said you do, then you dont have to own her to prove it.
let that girl go and move on with her life on her own and learn to love her without owning her. keep that love for her in your heart and move on with your life while you let her move on with hers on her own.