Sometimes motivation can be hard to find and hard to get. Some days you do not want to get out of bed, go work out, go to work, read a book, do homework, ect for different reasons based on your life.
Mark Twain once said "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover." This quote has helped me to stay motivated and go after the things that I want.
When you look inward figure out why you are trying to go to the gym, for example, is it a physical motivator (trying to look good for summer), a spiritual one (finding yourself), or an emotional one (getting over a break up). Look at why you want to go to the gym, why you want to work out, and you will find your motivation again-I know I did.
When you look outward it is to find support. If you are going to the gym you might go more often if you had a buddy to work out with and you guys together were getting ripped. If you signed up for a competition (run, spartan, body building, ect) to motivate you to continue going. Or even telling someone that you are going to the gym and will go 3 days a week because they can help keep you honest.