I am traveling to Japan next year. Until then I want to be able to order a bowl of rice by myself, even if my Japanese is awful. That keeps me motivated. 

Also I concentrate on the things I achieved and don't blame myself for having trouble in some areas (kanji!!! my worst enemy!!!)

what motivates me is probably the fear of embarrassing myself if I spoke incorrectly. Which I do all the time. I want to speak fluently so I can work and live in Japan without embarrassment!

I find that at the beginning few months, the motivation is for the language itself! But then as you get into real authentic content (novels, TV, speaking with natives), suddenly the motivation is simply in the content itself! Theres always another book I want to read, or show I want to see, and so learning the language almost becomes a side-effect of just spending time with that interesting material! :)

Bad book translations.

Try to find something that you're really interested in, preferably not anime or those aspects of Japanese culture, but more of a fanbase. Also almost every anime fan I know is really weird and not Japanese, fanbases are more common in Japan so it will also allow you to make friends with fellow fans and practice your Japanese. Getting into fanbases allows you to become interested in Japanese TV shows as they often make TV appearances. Watching Japanese TV will help you to learn more. 

Personally I wasn't taught Japanese growing up, though studied it during middle/ secondary school I learnt most from watching Hello!Morning as I am a huge モーニング娘。'15 fan. Still am haha. It should also help you with your accent as well. There are a lot of styles of Japanese music so hopefully you can find something you like :)

Having interest is one of the key things of learning. Similarly to a school core class/ subject, as it's manditory to learn those, you're not always motivated to do it as you have no interest (appose to the classes you choose yourself).