At the start of anything you can be very excited and energized. When you need to roll up your sleeves and dig in, maybe not so much. My mother told me many times as a child that I didn't finish things (if you know about astrology this was a Virgo to an Aries!).
What I've come to realize is we all need to keep the energy going to see anything through!
Since July, 2011 I work-out every day and at the end of the year I accomplish this goal at least 95% of the year. To keep me energized I work with a personal trainer once a month; she's also a personal friend and this relationship keeps me energized as well. It's like I have a personal commitment to Jessica as well as to myself.
Instead of criticizing yourself for not following through, start asking yourself what you need, so that you're regularly and consistently energized, renewed, and excited about what you're up to in life. Then put those ideas into action and notice what happens!
Here's to living your best life!