If you have access to a basic "dumb" trainer or stationary bike, it can help because you can just dial up a certain level of resistance / simulated incline and leave it there, so you can focus on your pedaling technique. I'm a strong-but-heavy low RPM rider, too, which is usually fine, because it's flat where I live, but I have to come up with ways to train, if I'm going to be riding anywhere there are real hills, or heaven forbid, mountains.

Try not to push down on the pedals; instead, focus on pushing forward over the top, and pulling back at the bottom. This works best on clipless, or with snug toe clips, but can also be done with a grippy flat pedal, although it's definitely more demanding on the technique. Don't worry so much about your ground speed, find your legs' natural cadence, and adjust the gears until you "just feel the pedals" (good quote ) you want to find that balance where you don't outrun the bike.