Why Do We Always Quit? Why is Change So Hard?

One of the biggest things people struggle with whenever they start trying to change something is nagging thoughts constantly reminding them that they're probably just going to end up quitting this like they quit everything else... and then ultimately those thoughts becoming a self-fulfilling prophecy when they end up quitting. This is especially true of people who struggle with roller-coaster dieting but it really pretty universal no matter what we're starting or trying to change. But does it really mean we're failures, doomed to keep repeating the cycle and never actually stick to anything? And what can be done to break the cycle?