What you're describing is pretty common. There are differences in the way landings are taught. Full flaps, no flaps; constant airspeed, incremental airspeed reductions until final. Most are correct, just different. So your best bet is more practice with your instructor.
But here's some ideas that might help.
If your base leg is virtually non-existent you are generally doing one or both of two things. One is simply being too close to the runway on downwind.
That's usually not the problem, though. Are you flying a Cessna? Are you trying to keep an eye on the runway during the base turn? If you are, then the chances are that you are in a very shallow bank and it's taking too long to make the turn. Assuming it's a left turn, before you make the turn, look to your left, pick a landmark to line up with. Roll cleanly into a 20º bank and roll out to line up with the landmark. When you roll out, you'll see that you are on base and in good position to visualize your remaining path to the runway and make adjustments.
The excessive loss of altitude in the base turn might simply be because the length of time you are in the turn, or it can be because of not maintaining pitch. Whether you are being taught constant airspeed from downwind to final or incremental airspeed reductions until final, each leg of the pattern has a target airspeed. And the best way to maintain it in a turn is to take the pitch attitude that gave you the airspeed on the leg and maintain it through the turn. Don't mess with the pitch or the power =during= the turn. You =will= lose altitude faster in the turn, but that's normal. Wait until the turn is completed.
Don't know if this will help. It's hard to work on specific things in an online forum, even if we could see exactly what you are doing.