In order to improve you need to continually adjust your training to get a training response.

By this I mean that what was difficult for you two years ago is no longer difficult now. Your body has become more efficient and so, in reality, you are stressing it less by continuing to do the same work outs at the same level of intensity.

People who always ride the same route will often fall victim to this.

You need to mix up your riding and join a group so that others will push you beyond your comfort zone.

Physiology also plays a factor. Perhaps you are better built for running but cycling on flat roads isn't your strongest suit on the bike. Maybe you would be a better climber?

Big, powerful riders will have higher ave speeds than you on flat roads but, when the road tilts up, smaller guys with a good power to weight ratio will come to the fore.

I hope that gives you some basic information to start with.