Motivation comes and goes. When motivation wanes, we tend to follow the path of least effort. However, new behaviours need lots of effort. This is because we find change really hard. The harder something is to do, the more motivation we need.

To help start a new habit, you'd have to be willing to be bad at it. Strip the thing you want to do into something so easy you could do it every day. It might mean going for a one-minute run or eating one piece of lettuce on your sandwich. You will start feeling a natural desire to do more when you  consistently keep at it.