Exercise instinctively

People quit their workout plans for one straight forward reason - their plan doesn't fit their lifestyle. Many people are too ambitious during first couple of weeks, and often will crash and burn from exhaustion. When you miss workouts due to fatigue, it's a good sign that your workouts are too intense,  but you need to know when to hold back. 

Recovery is vital for success with any workout program. Get your sleeping and diet in order.  If you can't manage to get optimum rest (don't work, personal chef and masseur) then don't go all out every workout.  Ultimately go with how you feel on the day and remember there will be a tomorrow. You want to be able to wake up in the morning without feeling groggy.  

You will often hear that anything less than 100% effort isn't good enough.  Truthfully if you were to give 100% of all your available energy to one workout you would be out for days unable to function in everyday life.

If you're doing 5 sets of an exercise then go to failure on just one. If you have had an utterly exhausting day at work then don't try to break your previous time on a run.  Save that until you are rested.

I used to know someone who worked shifts, which meant some days/nights he would only be able to get one hours sleep. However his workout was a three day split and he would stick religiously to it. This meant if a tough workout fell on a Monday, for example, and he had just an hours sleep, the workout would go ahead as planned even if he could have delayed it until the next day when he could have made eight or more hours. Insane. Needless to say his progress wasn't as good as it could have been.

So get your rest and tailor your workouts and days accordingly. Go all out, but use it sparing and you will have longevity in your fitness pursuits.