I do almost all my training indoors. I have gone back and forth between trainer, rollers and now a smart trainer. On the weekends, I do ride outside, unless my plan calls for something that would be better accomplished inside.
For me, I focus on just doing my intervals. Just riding inside doesn't work. I cannot maintain effort. But if I have a target wattage, cadence, gearing, or time, I focus on that.
I also watch TV. I don't Zwift. Some people give me crap for the amount of TV I watch, but honestly, 90 percent of it happens when I'm on the bike. I just run through TV shows or movies. I generally like stuff with action or intense drama. It helps keep me entertained and engaged in the show, and not my discomfort. Comedies and live sports are miserable to watch on the bike.
The one issue I have with riding indoors only is I have developed some bad habits, most notably, I look down when I go hard. This is because I end up watching my computer so much on the trainer, my head naturally drops when it's time to do an effort. It's something I've been working on a lot lately.