Hello,
I know a lot of people who are doing fitness to lose weight. It is a great way to decrease the amount of fat that you are carying, yet it's hard to keep it up.
There are a lot of reasons why you are losing motivation. The link points to an article that explains a lot of reasons why you might be losing motivation.
Article: http://the-truth.com/mindset/the-truth-about-motivation-why-you-cannot-be-bothered/