You need to recognise that if you're not feeling motivated, its not just your physical energy but your mental energy as well. And mental energy becomes drained when you (whether you realise it or not) are avoiding stuff that you really 'should' be doing. Essentially what I'm saying here is that its always a lot more difficult to motivate yourself to do things you should be doing opposed to the ones in which you want to be doing.
So the first step is to become more aware of what you're really avoiding.
Do you tend to be prone to procrastination or even perfectionism? Do you regularly find excuses for it to be not quite right to jump in and have a go, yet? Does your mind keep finding a way to distract yourself from making a real lasting commitment? Do you keep telling yourself that you simply 'can't' do this? Not yet, Not until you're ready? Funnily enough it's the people who procrastinate the most that are most likely wondering what's the best way to get motivated!
The problem is, if you wait until you're completely ready, it will probably be too late and you risk loosing out. Plus, that energy that you should be spending doing the stuff your avoiding tends to build up inside. And if you keep avoiding it, you end up feeling drained, lethargic, easily fatigued and generally low on core energy. After all, humans evolved having to fight for their survival, not sitting around being comfortable and watching TV. So despite our modern lifestyle which certainly gives us the comfort to conserve our physical energy, it ends up draining us of the emotional energy we should be utilising by constantly being challenged outside of our comfort zone.
That's not to say that it's a good idea to walk blindly into the face of the uncertainty just because you're fed up with your current circumstances. Planning and preparation are still very important. But sometimes it's hard to know how much preparation much is too much, and when you're just making excuses, because it can be such a fine line between the two.
If you want to know how exactly why you have been struggling with the motivation you need to tackle life head on, definitely give this book a read.