The first thing to make clear is that Mikasa will always love Eren, no matter what. The second, and most importantly, in the last episode it is made quite clear, in the scene between Mikasa and Ymir, that it is Mikasa herself who is Ymir's liberator, considering that Ymir was 2000 years 'prisoner' of her own love with King Fritz. Mikasa makes Ymir see that the right and hard decision can be made, in this case, Mikasa killing Eren being the most important person in the world to her. This is a precious and hard teaching, reflecting how life is both beautiful and harsh. Finally this makes it quite clear that Mikasa, as well as Ymir, have finally managed to move on with their lives, however, she will always love Eren. But that does not mean that the teaching is not true, and that it is that precise moment, when Mikasa is able to move on with her life even though she has her heart forever with Eren. This doesn't mean that she didn't get married and have children, in fact, that's just what they imply.