But most of the times people know.
I think everybody knows but ignore those feelings that are laying in their guts. We all have to be more in tune with our bodies and feelings.
Not saying you shouldn't feel down or that your feelings are invalid, if I'm wrong please correct me, but I think you are feeling disappointed that you knew something wasn't right and ignored it.
A lot of times people ignore the flags and instincts that make us. It takes time to develop those senses but they will help in the long run.
That's why I believe that no break up is a surprise. Your first love, I will totally agree but after that initial heartbreak and if you take the time to reflect(both men and women) you will see the signs that you willingly ignored.
Also I don't think anybody in this thread is advocating staying with someone that is emotionally abusive or cheating because those are symptoms of extreme pain in someone's life. What I will say is not everybody is out here to get you. There are still some genuine people on this rotting planet but if you don't know you or love you, you will never see them.
One has to heal and observe and enjoy this year thing called life.
Relationships are not for everybody but companionship is.
No matter if it's just friends, fwb, whatever, you need some sort of love, even if it's just for life.
Break ups are easy, living after is hard.
Don't let hardships jade your eyes, we are all still too young to just give up.