Here's The Best Advice From A Single Guy Who Spent A Year Interviewing Couples (article link)

“One woman in Georgia gave some pretty amazing advice. She and and her husband have been married for over 60 years, and after being asked what her best relationship advice would be, she paused and said…‘Don’t be afraid to be the one who loves the most.’” 

http://www.businessinsider.com.au/nate-bagleys-best-relationship-advice-2014-2

"If equal affection cannot be, let the more loving one be me."
- WH Auden
(Quote cited from user22013 on the original Reddit thread)