Generally, it's easy to see something that has great momentum. By definition, momentum is the product of mass + velocity. Think of a rhinoceros. It takes a long, prolonged force (or energy) to cause that animal to pick up speed and gain significant momentum. Equally, it takes time and energy to stop the momentum once it has begun.
Many times, we seek momentum in our day-to-day, and month-to-month lives. While we work, it's ideal to have a little momentum and get a number of things accomplished all at once. Having momentum in a daily exercise routine is the key to longevity. And we all know once the momentum slows down, it's difficult to start again.
But there's another kind of momentum that's not easily spotted. Rather than mass + velocity, I see it as mass + apathy. Call it the sin of omission or neglect, but when we don't do something for an extended period of time it seems to gain a sort of negative momentum.
Just as a runner who goes out every morning, it seems easier and easier to get up and run. There even comes a point when you enjoy it. On the flip side, the longer you abstain from an activity you're supposed to be doing, the harder it gets to start.
A rhinoceros that has not "charged" in a long time will find it very difficult to get up to speed. Watch out for the things you know you're supposed to do, but neglect them. The longer those things are neglected, the more negative momentum they get, and the harder it becomes to start.