Ok, so I'd get kinda hating your best friend a little if it feels like he's stolen every opportunity at a relationship you've ever had, like you just keep getting blue balled and baited over and over again, I'd have some mixed feelings about the guy too. Definitely a sucky situation to be in all your life. But.. I don't know if that's what's being implied. And even if it is, have you not had the guys sister pining after you for your entire life? Do you have mixed feelings about her by association? Is the thought of dating your best friends sister so messed up to you that you don't consider it? Do you just not like her like that? I seriously doubt it's that last one cause you seemed VERY ready to date some stranger who you thought made you a love letter. If he does hate his friend and not just the situation that his friend accidentally put him in then I think the author didn't really.... think this character through. Like, unless he just does not under any circumstances consider his childhood friends for romantic partners, he absolutely has options (Or at least a option) and therefore his life should not be anywhere near as miserable as he's making it out to be. Not gonna blame him for comparing his life to a side character in a manga though if he's actually been "NTR'd" as he would likely put it for his entire lifespan.
Or maybe I'm misunderstanding and it's not that he's gotten cucked but that being compared to his friend is the problem. Which like... ok. uh. did you talk to the guy about that? Cause at that point, if you weren't even interested in the people that are pining for him, then I think you're just letting random peoples opinions get to you too much. Like yeah it sucks that you feel so inferior to your friend and stuff but he's nothing but supportive and caring to you supposedly so it REALLY feels like this is just a you thing and you just have an inferiority complex and at this point even if you were to completely leave his life you'd probably just find someone else to compare yourself to and start feeling sorry for yourself and saying you have terrible luck all over again. Idk I'm yapping but he can just like talk to his friend about this and everything would probably work out.
But hey, you got a yandere now! Issues solved right?