It's mostly me not knowing the strings. Such a big roster which is amazing. I love it. But I feel like I picked up the game too late... I can do combos, I can mixup up decently... It's just I don't know what to punish and what not to punish etc. So I get dominated when the pressure is on...
I do a lot of practicing. A LOT and do/complete a lot of the tutorials. Mostly the combo trials. I search YT for combo comps and study top players but if it really is me just being new to the game, I just need to suck it up and study the characters I play more closely... I can't expect to instantly be amazing at the game...
Try picking up Hayabusa. Master has a ton of Busa tech on his youtube and he streams often as well. Will set you for life with that character.
Other than my shameless HayabusaMasterRace promotion my best advice is to fight against good players. Not ones who just pull fancy combos but players who understand how the game works.
Trying to be overly clinical with punishing may be part of the problem. There's a lot of shit that *should* happen in this game, that *does* happen offline.
And then you go online, and the rules change every game due to the network conditions. Things that should connect, don't. Strings that should chain miss their timings.
Reading the situation for what it actually is, is just as important as knowing the theoretical, and knowing what the offline game says you should do next.
Finding that gut instinct to distinguish the two comes, as @Kyo said, purely from exposure and practice. Can't learn in this game without losing. Chin up, and get back to it.
Knowing the theoretical doesn't count for shit when you've got a Ryu tele dropping you, and the network is so damn slow you can't even begin the sidestep, let alone complete it... And that move literally has a millennia long spin up.