I'd argue she will be happy actually moving on this time. This is really a bad way to see it I think, Lemon's problem was more related to how she had ended things with Sunao than really about not being with him. Of course, she wanted to be with him again, but she didn't feel entitled to it, and once she confessed and got rejected, she began to move on quite quickly... I interpret this really that the real reason Lemon still wanted to be with Sunao is because when she wrongfully ended things with him, she still loved him but distanced herself, so she couldn't move on. She needed to get closure both of her mistake and of her feelings.