Hi! Like you, I also crashed higher chemistry last year and I am currently doing advanced higher.
I know this point gets repeated to DEATH but do past paper questions - lots of them! I had failed all 3 of the topic tests before my prelim and was estimated a D, however, after doing a ton of past paper questions I got an A! Literally every higher chemistry question is a reworded version of another question.
Additionally, if you're finding the theory hard to grasp, focus on the calculations, then come back to the theory. If you can make sure you're able to answer most calculations, you'll net yourself a lot of marks.
Finally, just don't give up. You got this, it's difficult - I mean, you're crashing higher chemistry for god sake! That's quite a feat!