Why is it hard for most people that are in relationships to accept that it’s over?

When my ex was distancing herself from me and treating me like complete shit , I held on to hope that she was just going through a tough patch in life and just making me her punching bag , hoping I would get my girl back like it was before , I was kind of blind to all of it and thinking does she want to end this relationship or does she still love me , I was with her for a long period of time so I guess I couldn’t grasp it was over between us even though she treated me like complete shit , It’s like I didn’t want to believe it was over so I held on hoping she would snap out of what she was going through and accepting my love for her again to find out she was cheating on me? Why do we as people hold onto someone that is clearly treating us like shit? Why do we feel most times that Love will save the day? When do we decide it’s time to be selfish and tell them to go fuck themselves instead of just sitting back being a punching bag for them? When do you really know?
Why is it hard for most people that are in relationships to accept that it’s over?
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