I had a similar issue when I started doing circuits - I'd suggest you ask your instructor to show you what the "picture" should look like on final and fly the POH airspeed. Once you've got those, it's just a case of "picture, airspeed, picture, airspeed," all the way down final. Power controls your height and elevator controls your speed, but in reality you'll be making constant changes to both in order to maintain your picture. Out of interest, which 3 axis are you flying?