That's your problem. Focus on either Stamina, or Magicka. The strength of your skills and weapons in dependent on those stats, so at the moment you are only half-efficient.
Chose one.
Then equip 5 pieces of the armor connected with said stat (light for magicka, medium for stamina), one piece of the other, and one piece of heavy armor. That way you can level them all up.
Did you not put any points in the skills from the other two class skill lines? Do so now. Always keep at least one skill from all three lines so you level them all up.
Are you only using one set of weapons? You can use two and weapon-swap.
Where did you find this "Sap Knight build", and how close are you following it? Because it sounds like it's very old, and not very suited for leveling at all.
A good place to find builds is Tamriel Foundy (check the date on builds though):
http://tamrielfoundry.com/
A good starting point for new players is Deltia gaming:
http://deltiasgaming.com/
His leveling builds aren't too bad and at least make sure you level up all class lines and armor types.