I've seen this film many times and this part still confuses me

Mitsuha just dont want Taki to be the one going out with Okudera, if that makes sense.

It’s been a while since I’ve watched it, but just guessing from all the times I’ve seen that maybe she’s saying she wanted to go because she was the one who put in the effort to set the whole thing up. Making it so Taki is the one that gets to enjoy it even though the whole thing, including their possible relationship, was her doing.

Another explanation could be that she doesn’t want Taki to go because she doesn’t realize it yet but she does have feelings for him and for him to go on the date means that he’s leaving her behind in a sense or moving further away from her. And then with her crying in the mirror she comes to that realization, which gives the viewer meaning to what she said. Looking back on it now I think the latter is more likely but I’d have to watch again to be sure.

I think that she just wanted to go on the date because she was having so much fun with her. And also maybe to seal the deal for Taki.

I consider that Mitsuha arranged that date thinking she was helping him get a beautiful girlfriend.
You know, being a matchmaker, setting up couples and arranging marriages is quite fun.

From what I see, she got so much into the mood of arranging the date for "a friend" that she did not notice her feelings until the next morning.
Noticing her real feelings, she runs out to stop him from going out.

I think Mitsuha also wanted to "date" Okudera since Taki - Okudera relationship was built by her, so she sure have a lot of fun moment with Okudera. She didn't mean "date" literally, she just wanted to hang out with Okudera. However, since her initial goal was to get Taki and Okudera to be closer, she told Taki to make the best out of the date and enjoy it

Sometimes you say stuff that's not reflective of how you feel with the aim of playing the social situation itself. "I didn't set this date up for you out of the kindness of my heart, I did it because I wanted to go. If you go, you're obligated to enjoy it for me" dismisses any sense of indebtedness that comes from receiving a favor, and encourages Taki to focus on having a good time without fretting over the context.

This sort of almost gamelike thinking ahead and steering of the social situation is common in real life too (e.g. "Oh I just happened to be in the area" or "No no, I'm so stuffed, please go ahead") but especially so in Japanese culture, which puts a significant emphasis on consideration for the feelings left unsaid by those around you. If you look for it in Japanese media, you'll find it quickly.

Like she is jealous and doesn't want him to go, so she in his body can make it so that "he" doesn't go on the date. So it's more like "I don't want you to go" than "I want to go" at least I think that's what it means... watched many years ago so don't remember well...