Why does it hurt so bad if you are the one that ended the relationship??

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After three years of being together and six months of living together, I ended the relationship with my boyfriend just this past weekend. I'm seven years younger than him and I told him I want to be on my own for awhile. Ever since we had moved in together it has practically been one fight after another so I told him I think its best that he move out. If this is what I wanted, how come it hurts so bad??? How come I keep second guessing my decision and how come I miss him soo much?? This is what I wanted and yet I'm so miserable. He just moved out and all he took was some clothes and his toothbrush so all of his stuff is still there. Is it too soon for me to consider asking him to stay or do you think I should give it awhile before making a decision???