Why halloween feels so lonely after the breakup

Halloween used to be one of my favorite holidays, it was the only holiday I could spend with my friends and with her. She used to always want to come over and spend it with me all the time. I felt that this was something special we shared together. Whether it be dressing up to walk around or staying in to watch shitty horror movies. It made me feel like, to her, she couldn't imagine spending it with anyone else. However this year, it's different...we aren't talking now(read other posts for more context) not only that but she told me how she had a hard time picking wether not to spend Halloween with her friends or her new boyfriend. Anyone but me... I should be happy for her, right? I shouldn't be so selfish in thinking this was something she HAD to spend with only me. I want her to be happy even if it without me, but why do I feel so upset? She should be able to go anywhere with being tethered to me. She shouldn't have to worry about how I'm feeling because it's my responsibility to not let me feel this way over something this stupid. But I keep remembering all the fun we had, and how we will never do it again. I'm going to be alone this year with no one to celebrated it this year. Meanwhile she is better off without me. I'm crying thinking about all of this when she has things to look forward to. After she hurt me, after I held this relationship together all by myself. I don't want these feelings to fester into resentment or jealousy. So I should just probably stop feeling this way.