心甘情愿一厢情愿
[xīn gān qíng yuàn yī xiāng qíng yuàn]
'Willingly one-sided love' describes a situation where one person falls hopelessly head-over-heels in love while realizing it isn't mutual at all. Yet despite knowing such things wouldn't happen as desired, they still accept all that happens with all heart willingly and take everything from the relationship as long as they're allowed to cherish unreciprocated feelings.