Motivation is just a feeling! Like any feeling, your motivation will fluctuate daily, if not hourly. You might start your day with a kale smoothie and a walk to the station, but end it with a Netflix binge and a tub of ice cream. So don't be upset if you 'lose motivation', because you are human and that is completely normal.
So how do we combat a feeling, how do we 'stay motivated'?
Make a plan, stick to it and make it easy for yourself! Let's use going to the gym as an example. The first step is to have a plan. Set a realistic goal of how many times you want to make it to the gym in a week (on that note, plan exactly what you will be doing at the gym on each day).
"make a plan, stick to it and make it easy for yourself"
Great, you have a plan. Now step two, stick to it! Yes, this is where we have to exercise will power and discipline, both of which are heavily determined by emotion. So step three, make it easy for yourself. Take will power and motivation out of the equation by creating easy steps that make your plan foolproof. If you plan to exercise after work for example, take your gym clothes to work. That way you can go straight to the gym and not be tempted by the couch at home. If you plan train in the morning, go to sleep in your gym clothes so you can just wake up and go. Make your plan as easy to carry out as possible. The less obstacles you have, the more likely your are to perform that task.
If you make a plan and stick to it (by making it easy) you will completely bypass motivation and stay on track towards your goal. Over time, you will be able to create a habit where this plan is second nature and you don't even have to think about it. We are creatures of habit after all!