At this point it’s up to you. Which story do you want to tell? The one where Mitch is fixed on something, or the one where Mitch’s attitude grows and changes? Assuming you don’t have the entire story planned out, you should be able to put things where you want them. I know when I first got into Dramatica that idea didn’t seem particularly helpful, but try to embrace it. It’s so much harder for me to analyze a half formed idea until I get the perfect storyform than it is to start with what I know and mold the rest as I go.
To give you what probably feels like a more helpful answer, this part
Talks about youth and vibrancy and the present, but it also talks about her ability to embrace life. So ask yourself, does she affect others by being youthful, or by embracing life? It’s up to you. I don’t want to steer you in any direction you don’t want to go, but if we’re talking strictly Mitch as the MC and Sadie as IC and not looking at OS, I’d want to go with Sadie in Physics (influence through embracing life) and Mitch in Psychology (a desire to be loved). But I could also totally see a story where Sadie’s state of being youthful affects Mitch who believes that Sadie is in love with him. The difference might look like a scene where Mitch causes conflict by assuming Sadie loves him vs Mitch causing conflict by giving Sadie a chance to show Mitch that she loves him.
Now, to make what I just said about Mitch being in Psychology a little more difficult, if we are talkin about the OS...
I could also see an OS in Psychology and it would probably be a pretty smart move as many love stories have an OS of Psychology.
A final note, I don’t know what you’re going for and, as I said, you can really take it any direction you want right now, but what stands out to me is Conceiving. Mitch, whether as an OS character or an MC, has the idea that Sadie loves him. I can see this idea being the root of a lot of conflict. The goal of the storymind, even though Mitch doesn’t know this from within the story, might be for Mitch to stop having that idea. That would mean accepting that she doesn’t love him could be Success if OS or Good if MC.