It's one thing to start, it's another thing to continue. It's even a much greater deal to complete what you've started. People are usually psyched when the commence on a particular thing; that new job, that workout, that course you just started learning. But as times goes, we find ourselves having lack of interest or enthusiasm towards these things.
It is very normal. At some point in life, no matter how exciting something may seem at the first instance, we're most likely to get wary and tired later on. It is one common trait among humans. Nobody can honestly claim never to get tired later at some point in time. So what can we do when we get to this point of being unmotivated?
The first thing is to remember the reason why you started off with what you're doing in the first place. This is the drive that can help you keep you on course. The "why" in whatever we are doing really matters a lot. When we are able to identify this, we'll be able to bounce back whenever we fell like giving up.