How to get better at classical (G/90+30) chess

Here's the thing... I'm decent at shorter but standard USChess time controls (G30, G45, etc). But my results in FIDE classical format absolutely suck. I think in shorter time controls, it's easier to play by intuition, there's more room for error, and I normally play well in mutual time pressure. While in G/90, it requires a lot more patience, and way less room for error. So I'm wondering what I need to do to get better at longer time controls. I want to play FIDE events, but it's gonna be hard to justify if I almost know I'm gonna lose points

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Finding a trustworthy training partner close to your own rating would be your best bet. You need to play longer time controls if you want to get better at them, and it's also obviously impossible to find these matches by random matchmaking.

If you have a local chess club irl that would be a good place to look also

If you have issues with playing well when time is short, how do you improve on that? Use the same idea for improving with longer time controls. If you calculate deeper than your opponent and your evaluation of the final position after your calculation is more accurate, you should be able to win that game.

I went a long way to tell you, in my opinion, you should spend most of your time practicing improving your calculation to as deep as you can and practice evaluating the same position after your calculation. You should be practicing on positions that are somewhat challenging to you so that you may be spending 15 - 20 minutes per position. What is the max amount of time some titled players spend on a tough position? That should give you an idea of how long you may have to think in some positions, then train for those long thinks.

I like Jacob Aagards grandmaster preparation series for this. His books are very hard, as you probably are already familiar with. But at your level, this may be about right for you. You'll need to spend many months of tedious work for this, and you'll have to realize you may not get to your goal when you want. Every time you sit down to study, tell yourself your goal is to become 1% better today!

You should also join a slow playing league and try and get at least one long game per week. Dan Heisman's group has people interesting in longer time controls last time I checked which has been a while.

Classical chess often involves more nuanced positions, where small positional advantages can be more significant than tactical skirmishes. Understanding how to handle these positions is critical.