Confused about him

My best friend and I were interested in each other. Ended it after a year because of going long distance. In consistently being there for him, my feelings have deepened but he has taken my presence for granted. I tried moving on emotionally but regular contact has made it difficult. Since then, he has gone through multiple girlfriends. After each breakup, I was his fall back relationship. He would act like he's with me and be vocal about us being together. But he would move on at the next opportunity and I would act like I was never falling for him during the time he didn't have a girlfriend. This cycle has gone on for 7 years. Have known him for 15 years. Right when I thought things might work out, he just got another girlfriend. I was young when we started liking each other. I didn't realize the value of space and letting a person miss you. Have I ruined the possibility of being in each other's lives? What should I do? Is walking away the only option left?

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