It's really hard sometimes to stay motivated, no matter who you are. There's the initial excitement of a new project or task but pretty soon the reality of schedules, deadlines and changes, (oh, the despised and dreaded changes) rear their heads and suck the wind from your sails. There is a quote that I love and often think of when I get into this situation, (hopefully I don't screw it up too badly).
"Being a Professional is doing your best when you don't like what you're doing."
It's a simple thought but I agree with it 100%. There are always those assignments that everyone tries to avoid but inevitably has to get done. I have been stuck with these, sometimes 2 or 3 in a row, and I'm often surprised at the turnout! Maybe just having the right attutude was all I needed or perhaps the task wasn't that bad to begin with.....