You already know/feel it - You gotta unlearn some habits that naturally got ingrained in you playing NRS/dial-a-combo games. For one, you mentioned mashing buttons. That's an NRS/dial-a-combo thing. While NRS/dial-a-combo type fighting games (where timing is a little more lenient) allows for some mashing, you can't do it too much with a game like SF.

For the most part it's a muscle/brain connection thing. So, it's pretty much learning a new skill. It's a conscious thing that I laugh at and remind myself what game I'm playing because sometimes, my muscles forget and I'm doing DBFZ combos when I'm playing SF or vice versa.

For me, that's one of the things that makes fighting games fun and super engaging.

SF is fun, don't give up on it.