This is my opinion but I think he realised he loved her during the chapter when he stayed at his fathers house for the first time with Tessa n developed true feelings for her🙃 Great question btw!

@Katniss4Tris I totally agree! I feel like that night changed everything for him!!

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I agree with @Katniss4Tris I fell that night change alot for him. he developed feelings for someone that cared a lot for Tessa

Hardin knew, that he felt something different for Tessa, something he never felt before, at the stream. But he KNEW he LOVED her, when they stayed at his Dads house together. That was the first time Hardin slept with anyone, considering his nightmares and stuff. He trusted her.