Practicing Instrument?

I currently play the cello, but I have a problem. I'm the sort of objective-based person that like to see results. After practicing for (insert amount of time) I just can't help but notice that after that amount of time I am marginally "better", if any, at the one part that I was practicing. It just seems like a time sink that I can never find progress in. It feels like running on a treadmill; you can run as "far" as you want, but when you get off, you haven't moved. I need help finding motivation so that I can be my best. Thank you.