Whenever you are struggling and wondering why you embarked upon your little research adventure, I'd encourage you to watch this speech by Steve Jobs.
Regardless of all the hype around him, this speech is short yet so powerful. Watch it during a coffee break, and then wonder what invigorates you more: the ideas and looking at the broader things in life, or the shot of caffeine.
As a graduate student, PhD researcher or early career researcher, you've already stepped out of some of the expectations of society, and put your learning and curiosity on the first place.
Never forget how giving that liberty to yourself is perhaps one of your best gifts to yourself. Cherish it, even when your bank account is running dry or when you have endless comments by reviewers. Keep searching, exploring and playing around.