I've read and reread this manga like 5 times and have only begun to understand it recently (I know I could have probably looked at this discussion but I just wanted to type my own thoughts out).

So Touko can't see herself fall in love with anyone, right? But she wants to be able to love someone because she's lonely. And on top of that, she thinks the word 'love' is a 'shackle'. She's received a bunch of confessions from boys and girls who only know her by her persona (which I will call sister persona), and for that reason, she can't reciprocate their feelings. And then Yuu comes along. Yuu's never experienced love and doesn't treat Touko special like everyone else does. In not treating Touko like she's 'special', it actually makes Touko feel special since Yuu is the only one who treats her normally. That's why Touko is able to fall in love with Yuu since the very beginning. Touko saying that she's fallen in love with Yuu is her finally placing her own 'shackle' on someone else as a way to not be as lonely. However, she still wants to appear 'special' to everyone else because that's what everyone wanted from her when her sister died. She's acted out her sister persona for 7 years, so when Yuu tells her that she should drop it, it makes sense for Touko to say that she'd rather die than hear Yuu say that. It'd be like if you spent 7 years trying to achieve something and busting your ass trying to do it only for someone to confront you and tell you to stop. You can't just stop, and neither can Touko. Now, is it kind of selfish for Touko to ask to love while not being loved back, on top of asking Yuu to stay by her side forever? Yeah, but in Touko's mind, she thinks it's fine since she thinks that Yuu will really never fall in love with her (which we all know is not the case).

For not understanding until reading it 5 times, I have to say that this is the most complicated Yuri that I have ever read!