Your Dog feels that way because you might not be giving her as much attention as she needs, we humans think that they are just animals so they dont really notice, but dogs are very sensitive they can tell when we are sad, in trouble or even sick.
Thats why she is sad when you leave her cuz she love you and want to spend more time with you.
Maybe when she were a bit younger you gave her most of or time but now that she is bigger you dont really do it like before and she is use to that.
Try training her to play at a certain time of the day just the way you trained her to potty and she will change her behaivoir, she will understand when its working time and when its playing time, and trust me she'll be looking forward to it ;)... Hope it helps
Ps: she looks like a labrador they are very loving and caring so its worse...