It's called a "trainer." It's supposed to be hard. I figure a trainer ride will be at least 25% shorter than a road ride for the same training effect. If you coast a lot, maybe 50%. So in a way that's a good thing. Who wants to spend 2 hours on a trainer? Get your workout, then get off and do something useful.

I wouldn't get a heavier flywheel or even think about complaining about it. A lighter flywheel actually is better for improving your pedal stroke. Yes it's harder, but like I said, that's a good thing.

One thing I suggest is changing how you ride. You shouldn't be in the big ring much out on the road. If you don't already, get a computer that will show cadence and try for a cadence in the 90 range on the trainer. Because of gearing, it won't be exactly that, but within 5 rpm say.