Someone who doesn't value and respect your time is undermining your sense of self-value. And if you have a Flake in your life, they show it on the daily. They constantly back out of commitments, and they expect you to accommodate their ever-changing schedule—all while getting pissed off that you won't spend time/effort on or with them.
Look, things happen. It's a busy life and we all have things that shift and change. But if they can't respect your time and honor plans and arrangements made, put it out there. Set your limits and boundaries in a very direct way. If they can't change their behavior, it's time to move on.