How do I motivate myself to build a browser game alone?
Break your big project into smaller sections and complete them one by one. It helps to motivate if you get something right. That's what I did. And then in the end put everything together. Also stay at it, it might take a while. You might also want to impress other people with your progress then they can motivate you too.
Okay, thanks for your advice @GreenGem :)