If you don't think Ford missed Stan at all, not even a little bit, during their ten year separation, he would have agreed with you, pre-Weirdmageddon.
No, of course he didn't miss Stanley at graduation, when they accidentally called out Stanley Caryn Pines after Stanford Filbrick Pines and he definitely didn't look for his brother to be walking across the stage, only to remember that Stan hadn't graduated, that Stan was gone. He didn't feel sick at the realization that that was it, that he wasn't coming back, and that things would never be how they had been again. Well, there was that saying about making and lying in beds. He didn't think about it.
He didn't miss Stanley at college, where most of the other students either completely ignored him or (on occasion) made fun of his hands. When he finally threw a punch at one classmate in particular, he didn't grin, waiting for a certain voice to call out "Get 'im, Sixer!" That hesitation was all it took for the bully's buddy to get a hit in, leaving him with a nasty shiner, but things got better, after that. He got a reputation as someone who wouldn't back down, who didn't need to hide behind someone else for his own shortcomings. It was... nice.
He didn't miss Stanley when he met Fiddleford, the only other person who understood him, who cared enough to want to understand him. He didn't slip up, either, saying "us" or "we" when talking about Glass Shard and the things he'd done as a child. He confessed to Fiddleford, eventually, after a long night of sharing smuggled beers in their dorm while studying for midterms, about having a twin whom he'd had a falling out with. Fiddleford had said he was sorry that had happened. Ford asked why, and didn't understand when he replied that it must be lonely. He wasn't lonely- he had Fiddleford. And himself.
He didn't miss Stanley when he moved to Gravity Falls, staring out at the giant forest, something he'd never seen in person before. He didn't laugh over his discovery of creatures beyond his imagination, looking to the side for someone to share in the wonders of discovery. He didn't step into a cave and call "Echo!" and wait for someone to yell it back. He didn't feel hollow inside, as hollow as the cave, when the only repitition was that of his own voice.
He didn't miss Stanley as he wrote in his Journals, sharing discoveries and adventures in the same style as he would if he were telling a cherished companion who was simply not present for it. He didn't miss practicing someone else's handwriting to cheat on homework or confuse their his parents. He didn't miss random doodles and sarcastic comments scribbled in the margins of his writing, evidence that someone else had read his thoughts and cared about them. About him.
He didn't miss Stanley when he met Bill. Why would he? He had a new Muse to do wonderful and important work for. He wouldn't have had time for Stan, even if he hadn't ruined everything. (Besides, if he wasn't paying attention, those comments of "Sixer" and jokes felt familiar, filling a void he never dared to acknowledge.)
He didn't miss Stanley when Fiddleford came to help him. He had another person in his cabin, and any potential loneliness he may have possibly been feeling disappeared. So what if Fiddleford's joke about them sharing bunk beds fell a little flat? He didn't know, it wasn't his fault. It wasn't his fault. It wasn't his-
He didn't miss Stanley when it all went to shit. He didn't miss the reassurance that someone was in his corner, he didn't miss the constant humming and tapping on things that would cover the incessant screaming in his mind, he didn't miss someone climbing into his bed after a nightmare to help ground him, he didn't miss the offer of a high-six, he didn't, he didn't, he didn't.
(It was only pure desperation that made him send for Stan. That was the only reason he would allow himself to acknowledge. Besides, he was being selfish even asking him to come- Bill would kill Stan if given the opportunity. Ford knew that. He would make it slow, in every way Ford knew Stan feared, because Bill had peeled back the layers of his mind, treading places even he hadn't set foot in since he was seventeen and closing a curtain. He couldn't let that happen- even to someone he didn't miss.)
Post-Weirdmageddon Ford would look you in the eye and tell you that twenty-eight year old him (and forty-year-old him, and fifty-eight-year old him) was full of shit, actually.