I am in the exact same position as an inhouse designer for a private business company. The work is mundane, but it pays so well. Coming from a crazy hectic and demanding field like the ad agencies I started out in, this is a cake walk. And like you, I also just signed a lease to a new awesome apartment closer work. But I'm just getting so tired of showing up to work day in day out.
What keeps me going is keeping in mind that if I do quit, I would be putting myself in hardship financially(not being able to keep up with student loans, paying rent for my new place, ect). But if motivation is what you're looking for, what I'm personally doing is trying to capitalize on my vacation days, and really looking forward to those adventures. As you've mentioned- personal work. I'll be splitting my two weeks PTO days for photography. One week I'll be taking a trip to New York just to do photography. Then later in the year, I'll be doing the same in San Fran. Now in order to fulfill this dream, I'll need to work in order to collect paychecks. So earning this trip financially keeps me going.
Outside of these plans, I try to fill up my social life to keep me happy. Work is work. I forget about this shit as soon as I walk out at 5pm. You guys have mentioned here you like to drink. Me and buddies like to smoke. So in the middle of the week we sometimes meet up, watch basketball on tv, get high, play videos, eat, and go home for work the next day. It breaks up the week real nice. Making night plans makes my day much easier too. It's the little things you know?
Another aspect that gets me through the weeks is knowing from experience that there are PLENTY of worst jobs out there I can be stuck at. Jobs that make you stay at work til 7pm, one that doesn't even pay enough, and ones with micromanaging bosses that literally stand over your back watching you work telling you how they want it designed. Whats worst than that?! And that's not us. So gratefulness sometimes over powers my complacency.
Sorry for the lengthy read. Keep your head up.