You are bonking.
You can only go so hard. If you are riding hard, you can only digest so much fuel, then after that it just sits in your stomach and maybe even makes you sick.
You can go for about 1.5 maybe 2 hrs tops hard, then you have used all available glycogen stores in your body. At that point, you will head into bonk territory if you keep pushing.
* Fuel up the night before – pasta
* Get a HRM and know how hard you are pushing yourself
* Get fitter so you can go faster without as high a heart rate
* Don’t overfuel during the ride (some of the advice here points to that TBH, and is bad advice)
* Regulate your temperature properly (better helmet / clothes)
* Drink sports drink during ride to hydrate, gets carbs, and electrolyte
* As soon as you finish, get some chocolate milk in you, then some proper food.