Motivation is an enemy disguised as a friend. Motivation gets you excited to the point you dedicate yourself to taking on a long-term activity, then it runs out on you.
Discipline is a friend disguised as an enemy. Discipline forces you to do things you don’t want to do, when you don’t want to do them, and it’s always there to nag you when you don’t do what you’re supposed to.
As others have mentioned, consider “why” you want to learn Japanese. Then determine your goals. Then throw away the goals[1] (because they exist only to temporarily motivate you), and develop the systems that will actually get you there.
Here are some examples of systems:
- Use WaniKani daily. Complete your reviews, and do lessons when your Apprentice count is below 100.
- Read a chapter from Tae Kim’s guide each day. The guide isn’t perfect, but it helps ease you into learning grammar.
- Watch a CureDolly video each day. Even if you don’t understand all the content, you’re becoming more familiar with Japanese grammar.
- Read a random Maggie Sensei grammar post each day.
- Read a little bit of native Japanese material, and look up the vocabulary you don’t know and the grammar you don’t understand.
- Do shadowing, which may improve not only your pronunciation, but also your ability to hear how Japanese sounds are spoken.
Those are just examples, and you don’t want to necessarily implement all of them.
“How can I do any of those if I’m lazy?”
Only you can resolve that, no matter what anyone says. I’d say anytime you’re being lazy, ask yourself what you’re doing.
Are you turning on the television and watching whatever’s on, whether you’ve seen that episode or not, whether you’re interested in the show or not? Stop turning on the TV when you’re bored.
Are you reading through posts on Facebook or another social media service that don’t actually impact you in any way? Install a program to limit your daily time spent on certain web sites (such as the StayFocused extension for Chrome). That way you can wean yourself off.
[1] You can still have goals. Just don’t depend on them to keep you going.
TLazy;DR: See this post.