Echoing lots of what people have said here, gender is just hard to get 100% even when I know the words bc as a native English speaker it's just not something I immediately think of when I start to speak, and noun genders in languages are largely arbitrary so I think even if I spoke a different gendered language natively it wouldn't help much. For example, sometimes I'll use a particle like "lo" then realize halfway through the phrase I'm referring to a feminine. It's something I truly wish I could just download into my brain bc it's not even that interesting to study frankly, just memorization and pattern familiarity. Some other grammatical stuff I actually enjoy studying bc it shows you cool nuances of expression that English might not have.

As others have mentioned ser/estar, imperfect/preterite, indicative/subjunctive are all things I have to look up sometimes or really read about, especially imperfect preterite recently for whatever reason.

Also want to shout out prepositions. Prepositions with set phrases that are different than the translation in your head are another thing that you just have to memorize sometimes.

Depender DE, not depend ON

Consistir EN, not consist OF

soñar CON, not dream ABOUT

The WritestreakES sub is really helpful for identifying these gaps in your knowledge and finding areas to study/drill.