You increase your level by increasing your skills, not by earning XP. So killing and questing won't have a bearing on level, just the skills you use.
If you're not leveling any higher then you've maxed out whatever your primary skills -the ones you're using the most- so you can't level any higher using them.
Try using other skills. Though around 50-55 is considered a sort of 'soft cap' where whatever your chosen primary skills tend to be maxed out.