Why do my lapels roll open no matter how much I press them?

Nine times out of ten that gap at the top button is the roll line sitting too far out on the body, not the collar length. When the roll line crosses too close to the shoulder seam or angles too vertically, the lapel has nothing anchoring it down low, so the top kicks away from the chest. Wool blend being medium weight makes it worse because there's enough body in the fabric to hold that gap open instead of flopping closed. Quick check: pin the lapels shut exactly where you want them to sit, then look at where the natural fold wants to happen. If the roll line on your pattern is more than about an inch above where the fabric actually wants to break, that's your culprit. The fix for coat number two is sliding the roll line down and slightly toward the armhole, and adding a bit of width at the lapel corner so there's more fabric to roll under.

That said, collar stand length matters too, just in a different way. An overlong stand causes the collar to buckle and wave along the neckline seam, not usually a clean gap at the button. If your stand were the problem you'd see rippling around the back neck or the collar refusing to hug the neck, which sounds different from what you're describing. For the iron fights on this coat: try shaping rather than flattening. Lay the lapel over a tailor's ham and press with the lapel rolled slightly closed, holding it in the position you want it to wear at. Steam, let it cool completely on the ham before touching it. Do that a few times and the wool blend will memorize the shape to a degree. A strip of fusible hair canvas or even a sliver of fusible stay tape along the roll line inside can stop it stretching out and drifting open during wear too.

For next time, one thing that saved me on my first self-drafted collar: make the undercollar slightly smaller than the upper, then ease them together. It forces the collar to curve around the neck instead of standing away, and it's honestly the difference between a collar that fights you and one that behaves.

This is interesting to me beacuse I had a jacket where the whole front just wouldn't sit right and it turned out my fabric was kind of stiff on one side. After I washed it the sagging got worse actually, which surprised me. Did yours do this from the first wear or did it show up later? I keep wondering if some of these collar problems are just the fabric deciding to misbehave after a while.

I'm way too new to drafting anything myself but I had a similar saga with a vest I made where the front edges gaped at the top, and hand-basting the lapels shut overnight after steaming actually helped more than I expected. Not a fix, more of a bandaid, but it got me through wearing it. Question for OP since you've clearly done more collar drafting than me: when you pin them shut like the first reply said, do you do that on the body or on a dress form? I don't have a form yet and I'm never sure if what I'm pinning on myself is even accurate.