Bewildered & Baffled

I have a wonderful job working for a wonderful couple, but conditions have gotten somewhat intolerable at the office lately. Another coworker and their outlook place me under a great deal of stress.
My coworker feels that I am foolish to always put my best foot forward, and says that I am just being taken advantage of. I have been informed on several occasions that I am not appreciated, and that is why this coworker refuses to give it their best.
This attitude is so disturbing to me because I am incapable of doing anything less. I am passionate about my work, and I am a firm believer that everything I do is a reflection of who I am. But on the flip side of it all, I find myself seriously overworked and frustrated, simply because as I am running about like a mad person attempting to get everything done, as well as constantly looking for better ways to accomplish my job with proficiency, my coworker is calm, relaxed, talking on their cell, and smoking.
So, as a result of all of this, I like my job much less today, than a year ago, not to mention I am beginning to feel bitter toward my boss because nothing is being done about it, and I know this is effecting our work relations. Please help me resolve this issue.