semirequited love
Semirequited love is love that is reciprocated in a meaningful way by the beloved, but there is a disparity between the perceived meaning or intensity of that love by at least one of the partners that prevents them from forming a primary relationship or life partnership with the other.
She realized she was doomed to semirequited love when the love of her life wrote her into his will, held her when she found out her mother was dying, and professed that he wanted her to be his poly secondary partner as long as they both shall live, but that he believes that there is a woman out there who will be a more perfect primary partner for him that he is holding out for.