It's expected that you'd plateau about this time of the year. If you're doing it right w/r to periodization, every year will have a plateau, usually called a peak period. It's all downhill from there is every direction. A peak is impossible to maintain - one gets burned out/overtrained. You can hold this performance level for a while, maybe a month, then it's time to ramp it back down, rethink your periodization plan, and schedule another round for next year's season. You should be faster next year than this next year if you plan it right. What's "right" is the problem. That takes either research and experience or pro coaching.
If you still have some events later this season that you want to do well in, you can try about a 10-day endurance block. Ride a long but moderate endurance ride almost every day. Really pile on the mileage, but go easy enough that you don't get overcooked even with the high mileage. Then take a couple days off and then do whatever intervals, hill rides, etc., you think you need.
eventually, no matter how you train you run up against your max potential physically. Having said that there may be something to be gained by refining position which may make power application more efficient and also make you more aerodynamic.
1) You have reached the maximum benefit from your current training and nutrition and need to up your game.
2) You are overtraining and need to increase rest and recovery.
3) You have reached your maximum potential in a given sport and/or age is becoming a factor limiting your progress.
When you hit a plateau, don't throw out your whole regimen or make radical changes just to make changes. Carefully examine where your deficits are: Is it endurance? VO2 Max? Strength/power? Mental focus? Pain? Once you identify the specific areas that need work, evaluate your nutrition, training and recovery strategies focusing on your weak area(s). From there you tweak > evaluate > tweak until you get the results you want. I've seen a lot of individuals panic when they plateau and leap into intensive training without a specific plan. This usually results in a short burst of gains followed by declines due to overtraining and/or stress injuries.