What do you enjoy when you do exercise? Is it pride in sticking to your plan for example? That would get boring after a while. Are you getting the endorphins or fun you need? Maybe it's time to mix things up and try a new form of exercise. I'll risk sounding like a broken record but for a huge variety of bodyweight exercises Mark Lauren's book is good, and do them intensely enough and they'll give you a motivational kick for next time.
Also, see good old Arthur in the signature below
The trouble with the rat race is that even if you win, you're still a rat.
I take life easier than almost anyone I know, but when I exercise I do it as though my life depends on it (which it does).
(Arthur De Vany)