Almost all human beings are motivated by just two things. PLEASURE and/or PAIN. In order to stay motivated you need to focus on either one of those two.
Lets take me for example. If I was to no longer exercise anymore, let myself go and just get lazy (which would be easy to do because just like you I sometimes really can't be bothered to exercise) I would be faced with great PAIN.
I would become overweight, unhealthy and depressed. Most of all I would be out of a job. How could I be a fitness professional and not exercise? So this is my motivation. This is my PAIN. I always work with integrity so if I'm telling a client to exercise then I need to be doing the exact same thing myself.
That is most of the time my motivation. Believe it or not my clients actually motivate me and keep me accountable and keep me in shape.
Lets look at another example. What usually motivates most people to get healthy and into shape again? .... A RELATIONSHIP BREAKUP!
There is two things at play here. They are depressed from the loss of the relationship (PAIN) and they either want to look good to pick up someone else or get there ex back (PLEASURE).
What happens a lot of the time is as soon as someone gets back with there ex again the training goes out the window. They are no longer feeling pain so there is no longer a reason to do anything about it.
First of all you need to figure out what YOUR goal is. Without that you have nothing to aim for and work towards. Trust me, when you have to get up early in the middle of winter to exercise you need something to get you out of bed otherwise you just won't.
You need to figure out what it will mean to you if you don't achieve that goal. For example for me it would mean I lose my job. It may mean you will die because you are very unhealthy or obese and the doctor told you "If you don't exercise you are in big trouble".
It may mean you end up injured because your goal was to run a half marathon but you didn't stay focused and you didn't train.
A goal is what keeps you on track. It also gives you a deadline. Its best to divide the really big goal into little ones to stop the loss of motivation along the way.
The goal also needs to be about YOU. It can't be about impressing someone else (although it can be apart of it, can not be the main reason). You need the goal to mean something to you if you achieve it.
The reason for this, if you are losing weight to impress someone else and all of a sudden they are no longer in your life you have lost your motivator and there will be no reason for you to continue.
You need to tell EVERYONE about your goal. Tell them you've started to exercise and are eating healthy that way they will ask you how you are going. This is another way to keep you accountable. If you stop you will have to explain to your friends and family why you stopped.
Starting is always hard. Once you get into a routine though you will start to enjoy it. You've just got to start. Start today. You are the only one in your way. I challenge you TODAY after work to go for a quick 20 minute walk or alternatively do 30 push ups. Start small and then gradually work your way up. TODAY.