Derek's eyes closed and he tightened his grip on her, too, when he felt Patty cling harder. He was just as afraid of the other shoe dropping. That she'd disappear from right in front of him and be sent home, never to be seen again because if she came from the world with the Flash and Stiles read the Flash in comic books, then Patty came from a world that was fiction in the world he came from. If she was sent away, that was it. That was half the reason he'd been so scared to let himself feel what he felt.
Hearing her say that she loved him too, even though he'd already known that she had was a bigger relief than probably she realized. It was just as likely, in Derek's mind, for her to want nothing to do with him anymore. But she still loved him and Derek gave her a little squeeze in an attempt to let her know just how much be appreciated that.
His hold loosened just slightly when he felt her moving and Derek looked down at her, offering a weak smile and a nod when she suggested he change his clothes. Patty started to tug on him to get him to head toward the stairs and Derek pulled her back in, dragging her into the kiss he'd been aching to give her for months. He poured everything into that kiss; his pain, his relief, his love, his apology. His hands moved to her face, holding her there gently, because he needed it right then. She could be angry with him, she could even send him right back out the door with his gym bag and tell him she never wanted to see him again, no matter how much she cared about him, but Derek needed this one last thing. She'd heard him say that he loved her, but Derek needed her to feel.