Get an HRM and if you can afford one get a bicycling computer/head unit (I prefer Garmin). If you can afford it, a power meter is a big plus. I use one and I can't tell you how much it's helped. There are days where I feel like I'm just crushing it and when I get home and look at the metrics I can see that there was nothing special about it. I really believe it is critical to know where you're at and to give yourself a baseline.
Whenever I feel like I've reached a plateau I like to take things slowly. I will never go out and try to set a new record, I will add 5 miles to my previous week's distance, do that ride several times and then increase the mileage again.
But most of all don't ignore your body. If it's telling you it needs some rest... take it.