when you're getting tagged are you bringing your hands back up after you strike or are you not? make sure you bring your hands back up to your head after you hit or attempt to
Slugger mentions something really important here. The reason is because in the beginning when you do bagwork or padwork, because those don't hit back it's easy to throw strikes and then don't bring your hands back to your head.
Next time you work on the heavybag, make a hard effort to visualize the bag is hitting back, and shield yourself after each combo you throw against it.
Also, use that sway of the heavy bag. As the heavy bag sways back and forth imagine it's an opponent advancing and retreating. When the bag is swaying away from you go in for the kill, when it sways back sidestep or pivot out, or back up and guard up