I'm having mental issues (lol)

I'm either totally psyching myself out and thinking I can't run this marathon because I'm going to suck, or else I'm over-hyping myself and thinking I'm going to pull off a crazy run well beyond my expected ability. (This morning it was 57F when I went out, though up to almost 70 before I got home, and I did 9 miles with 5 at MP and those MP miles felt so good I thought, heck yeah I've got this in the bag. But realistically . . . I do not. And that was 5 miles lol)

Any suggestions for, I don't know, an actually healthy mental approach to the marathon? I've run so many shorter races (up to half) and so many miles in my life since I was a kid, and never really felt this before. (Well, I did go through a phase in high school of thinking I would somehow bust out an amazing cross country race when I hadn't really done the training to expect it, so maybe it's similar.)

Or is this more of a r/running question? :)