As you get fitter, you'll be able to do the same amount of work at a lower heart rate. Your heart is a muscle like any other, and as your fitness improves, it will be able to pump more blood per beat, so for the same output it will need fewer beats.

I have learned that it boils down to your 'pain threshold' ... especially when you are climbing hills or riding with strong wind blowing towrds you :p

Several things helped me, back when I was still trying. A big one was mental, trying to focus on maintaining level of effort. This is not just cadence or even heart rate. It's watching the average speed on your computer and trying to keep your speed above it, or looking out for landmarks and whether you have come to them any sooner, logging your ride so you see what your new goal is, lots of little tricks like that.

Some racing games have a "ghost car" that is on your fastest recorded lap, so you can see whether it's ahead of you. I think Strava does something similar but I haven't used it that way.

If it's about outright speed and not just your fitness, you can always look for ways to shorten the course or make yourself more efficient. I recently went to one of those electric gokart places and my lap times went down nearly a second when I figured out I could lift and slide around the corner at the end of the straight without braking.

If all you want is to go faster, there's nothing like a race to motivate you. Find someone who is faster than you, then try to catch them! Better, yet, have them chase you! With a cattle prod!