Should I tell friends that treat me like a backup plan that "I'm thinking if we should continue to be friends." or just drift away quietly?

If your friends 'treat you like a back up plan', those aren't friends at all (trust me, I experienced that alot and once I chose to push them out of my life, I actually made long-term friends). I honestly would just drift off and if some of your friends actually make it an effort to try and connect with you, that will tell you who your friends are. As hard as it is or may be, it will seriously be worth it : )

yes