I am in no way qualified to give general training advice, but having made the transition from slow, to significantly less slow (still have work to do); these are a few of the things that have worked for me. Of course, saying them out loud is a lot easier than actually doing it, but here goes:
1) Get a bike computer and associated sensors. It’s much easier to understand when the “suffering” is cosmetic or real with your metrics in front of you. If it’s cosmetic, ignore it. If it’s real, slow down. It will be hard to know the difference at first, but using your heart rate zones is good general guide. Spend as significant time just under the “real” line. Do your best to work through the cosmetic, the pain never goes away you just get used to it. Lather, rinse, repeat… Over time the amount of effort required to get to real moves up. As it does, you get faster.
2) Ride with people who are faster than you. Not getting dropped is a massive motivator for me to push harder. Having a defined target for “fast” is a huge help in getting faster. Being “forced” to keep up also reduces RPE, it’s much easier for me to push close to the red-line when competing (or less generously, posturing ;)) on a group ride, than riding solo. That’s not to say that there isn’t a place for solo rides, those are super important too. Just that it’s nice to know what you’re training for.
3) Put in the miles. This works for everything, not just getting faster. Want to lose weight, get stronger, build endurance? Put in the miles. They don’t have to all be crush your body miles either, actually better if many of them aren’t. Spending time on the bike whether it’s base, endurance, or CRUSHING IT pay off. You will get faster, lighter, stronger, etc.