Do i let my friend move on?

One of my friends,alex, has been one of my bestfriends for almost 3 years now. We have known eachother since we were 3 and now we are 15. we became really close 3 years ago. we have had some issues in the past though. she met a new bestfriend last year and kinda left me. i became good friends with alex's new bestfriend this past year, but alex treats me bad. she excludes me and only texts me when she needs help with school, or when she and taylar (her bestfriend) are in a fight. basically she uses me, but when we actually hangout together (which is rare) things are great! she acts like my bestfriend again. about a month ago, alex and i got into a fight. i finally stood up to her about everything and she felt terrible. i sent her a long message about everything, and she promised me things would change and we would be bestfriends again. i actually believed her, and for about a week, things did change. but as the summer has started, she never talks to me. this past week she invited me to go to dinner with her, and i couldn't go the night she wanted, so she kept asking me to go on different nights. i never had time to go with her, but i was flattered that she was putting effort in our friendship. then i realized last night, the only reason she was inviting me was bc taylar was out of town. taylar got back last night and alex wouldn't answer any of my texts again when i wanted to go to dinner. basically she uses me. i'm her friend she goes to when her bffs can;t hangout with her. my parents and other friends warn me about her all the time and i know i shouldn't be friends with her, but i do love her. if i sit down and talk with her i know she won't take it seriously and i just don't know what to do anymore. do i let her go? please don't tell me to do that because i can't...any other advice???