One must have a hell of a discipline to be able to become a successful startup. How do you keep yourself in check when it comes to things that you're the only boss of? I can't do something meaningful that I don't have to answer to someone about. Did you ever find yourself in a similar situation?

Oh, absolutely! I probably shouldn't admit that so excitedly.
For me, I always keep in mind why I'm doing this and what my long term goals are. I make sure to dream big for long term goals, while actually planning steps to achieve them, so I get excited. I question what my options are, and ask if doing this business is really the best option I have. Time after time, I decide it is. It's hard to work at all when you don't really think what you're doing matters.
I've found that it's been really important to analyze what exactly gets me at those goals, and what's okay to slip on. As a business owner, especially one who doesn't want to have a ton of employees, I can't do everything. Tax paperwork slips (oops). I respond to a lot of messages very late or not at all. I'll miss entire business opportunities, because there's just too much on my plate. I'm not on top of a whole lot of things.
Part of the reason why I'm not on top of many things is that one of my goals and a reason I'm doing this at all is I cherish having work-life balance. It's purposeful.
I make sure to always do things I feel are fun, and exciting, along with tasks that are important to keeping a roof over my head but boring/dreadful. Even if the fun things don't seem as crucial. It helps remind me why I'm doing this at all and stay motivated.
And being purposeful like this-- realizing that I'm the one in control and I don't have to just trudge down a boring to-do list of life, probably seems obvious to a lot of people but has been a revelation to me. I can not do a lot of boring "important" things I hate and still be fine. I can do stuff for fun and be fine. I can rest and be fine. This realization has made me much more motivated and energized for the tasks I do do.
An important note to add on this: work to be very honest and realistic about what actually needs to be done, and the impact it has if not done. Keep a cool head.
I'll say the obvious, too: Being healthy helps. Take care of yourself, and focus becomes less of an issue. Your body will suddenly want to do more than just rest a lot, your mind will be healthier. Try cutting back on coffee-- you shouldn't *need* it, and sometimes relaxing is far more energizing.
It's also worth noting that leaning on others is powerful. I keep myself active in the Seattle community and know a lot of entrepreneurs, and talking to them helps me feel less alone and also provides a wealth of insight into whether I'm doing the right things and what my end goals can be. I lean on friends and family. And of course, I lean on my business partner for feedback and leadership. Friendship, whether it be through others in your community or your supportive friends or business partner, has been a surprisingly powerful force, and also makes things feel more worth doing in general. Life is about good friends, right?
Anyway, I'm not sure if that's helpful and can definitely go into more detail, but those are some things I've learned.