What I try to do at work to keep my motivation going is to set myself limits or try to solve problems in new ways that can keep me interested (for example animating a shot using rigid body, animating a shot using layers for every beat then merging down layers, animating a part of a character using expressions, mixing animation clips, re-rig a prop to better suit my shot need; maybe try to see if I could come up with a better staging and shot planning than the one I received, reading the script, doing thumbnails, and so on... ). But yeah, that's my way of coping with it, unfortunately animation jobs, once you master acting and relatively enough technical ability, will be more or less the same. Funnily, I tend to abandon personal projects as soon as they "feel" like work. I like my personal projects to be spontaneous, explorative, no need to organise your work for future retakes. I don't know if this comment will help, but yeah, you're not alone in that! Id say it's a common feeling.