What he says is “I wish we had more time.” before he tells her he loves her and literally runs to his death. That’s it. Nothing that could possibly make her feel guilty, nothing that could have seemed like he regretted anything. Not “I wish we hadn’t gone to the front.” Just “I wish we had more time.”
And he then, metaphorically and literally, gives her more time. Because he knows his clock has run out, but that doesn’t mean hers has to… so he runs and saves today and gives her his watch. Gives her time. (x)