Ahhh, novelwriting. Yeah, it’s a lot different from finishing a few pages of fanfic, that’s for sure.
Completing a project that big is about discipline. If I let myself “not feel like it”, I’ll usually be lazy and watch TV instead. So make writing a routine. Hold yourself responsible every day (with reasonable exceptions for school/work/travel/family/illness). I find it helps to have a quota, like 200 words per day or some other goal that’s easy to meet. Working toward small goals is more encouraging than struggling toward one enormous goal.
A lot of people find it helpful to have a place devoted to writing. It trains you to get into a writing frame of mind as soon as you go there. It can be a corner of your house if you like peace and quiet. I go to a nearby coffee shop for the ambient background noise. I’ve been going there for years, so the employees and regular customers all know I’m a novelist. It’s a great motivator to have people say, “Wow, a novel? That’s cool!”, but on that note, don’t work in public if you’re not prepared to have nosy, well-meaning strangers talking to you.
Also, give yourself permission to suck. The first draft is allowed to have boring scenes and lame sentences and plot holes and spelling errors. That’s okay! You can edit later! No one writes a novel perfectly on the first try. The fastest way to kill your writing drive is pressuring yourself over every word, so just worry about getting to the end of the first draft. Then you can look the whole thing over and see what could use a rewrite.