Afraid to loose my bestfriend

Hello,

Me and my best friend have been friends for 6 years and have done everything together, going to the doctor the dentist shopping traveling together even live together it's almost as if we're married. I have become very codependent and attached to them. They love me a lot but they don't see me the same way. They've even done more sacrifices for me that I have done for them. But I have confessed and they don't feel the same way but also don't leave. They recently started talking to a long lost friend that they cared greatly about. And might even date in the future. And I can't handle what will become of me. They've been texting non-stop for 3 days straight. And I can't control my feelings. I turn toxic and ask a million questions. I know very well that I'm being toxic. But I also know that I can't stay if I actually love him it's not healthy for either of us. But we live together we do everything together and abandoning him for something that he doesn't deserve hurts to even think about. He tells me that they're just friends and he'll never leave me. But I know that in life once you marry someone and start your life with them, best friends end up on the side. It hurts me thinking that they will replace me and do everything that we used to do with that other individual. So I can't leave but how can I stay? Do I have to put myself through this grief. Do I have to show up to their wedding? They has been abandoned before and I've been with them ever since. The thought of me abandoning them is extremely unfair and cruel. I've talked to him told him this exact paragraph. And he always says that he won't replace me that he will always be here for me and even if he does get married and everything he will still try and hang out. My heart can't take this. To make things worse my family and his family are very close and we see each other every day and every weekend through social ties. Everyone sees us the best friends ever. So if I were to leave him everyone would notice. The only time I'm happy is if they're single because my life then feels like I'm married and everything is ok. Even though I know it's a delusion. But he won't leave me... And I can't leave him. So what do I do? Anyone been through this situation?