Yeah, it will only get worse. What you likely need to do is adjust the cable tension at the rear derailleur. That can be accomplished by turning the barrel where the cable goes into the back of the derailleur.
Screw the barrel in to lower the cable tension, and unscrew the barrel (screw it out) to increase the tension. You'll likely have to do the latter, since the cable is stretching. Turn it 1/4 of a turn and try it out until it feels right.
It also could be that you're derailleur hanger, where the derailleur attaches to the frame, is bent. It's a replaceable part on most bikes just for that reason. This would be easier for the bike shop to miss, so if you can't get the derailleur dialed in, that's you next suspect.