Tiredness after long rides
I am a long distance rider.  You don't sound unfit.  A few things to look at:


Diet.  Eating the correct food at the correct times greatly affects your performance and recovery.  Your post-ride recovery drink/ meal is as important as the ride itself and should be taking within 30min of getting off the bike.  Remember you're always eating for the next day.  
Sleep pattern.  Consider whether you're getting enough sleep.
Heart Rate zones.  Are you pushing too hard too long?  Doing a 200km ride at 80% of max compared to doing it at 65% will also have a huge impact.
Health.  Are you getting or sick?  Maybe you're showing early signs of a cold or flu.


Otherwise get off the bike for a few days.  It may also be psychological.  Then start again with a lower HR, focus on nutrition and remember to have fun!