All the advice you have gotten (above) is very good. Basically, keep riding lots and you will see quick improvement. I have to wonder if part of your "problem" is your pedal stroke. A trainer should reveal pedal stroke issues pretty quickly. Basically, there should be no "dead spots." Instead, you should feel like you are pedaling in circles with tension on the pedals much of the way around the circle. Are you using clipless pedals?

Also, I have to wonder about your being in your biggest gear. I'm not sure about your trainer/zwift set up, but on the road you would only want to be in your biggest gear on fast downhills. I can't help but wonder if the funny gear/low cadence is actually throwing your calibration off (on some trainers, this would be a problem, but I don't know about yours). Regardless of the calibration question, turning a monster gear at 60rpm is not going to be very practical for cycling on real roads.

Instead of jumping from 60rpm-90rpm, I might suggest doing some "cadence drills" where for 1min you switch into an easier gear and try for 70-80rpm, then back to your self-selected cadence for a few min. Repeat for an hour. Keep the power numbers the same, or similar, throughout the workout. Just settle into a power number at which you are working, but can still sustain comfortably for an hour.