My little guy loves me and my boyfriend. He is more cuddly with me and gives me kisses (he won't give my boyfriend kisses
) but he loves to play with my boyfriend and even goes to the lake fishing with him. He just has a different kind of relationship with each of us. I think you should try to have your boyfriend do some special things with her, like teach her a new trick or a new game, feed her, bathe her, take her with him somewhere... and that way they can develop a special relationship apart from the one she has with you.
She'll still think of you as her special mommy and may still have a preference for you, but she won't be as hesitant to also love be with her daddy. Good luck, and welcome to yorkie talk!!!!!!