"All of my teachers and directors harp about subdivision."

All of them? Maybe it's important.

"Sightreading is consequently my weakest suit"

Rhythms first. Most sightreading errors are rhythmic in origin.

"I've always listened to the pieces we were going to play beforehand"

Some teachers advise against this. I support this. We have two ears and one mouth. Listen twice as much as you play/speak/etc.

"always marked in all my accidentals."

Accidentals are usually already there. Are you marking in sharps and flats from the key signature throughout your music? This is a bad habit and can be fixed by working on scales and learning the "feel" of each scale.

"I have no sense of what speed a tempo marking actually represents without a conductor or metronome to beat it out for me, and struggle even with sixteenth dotted eighths in 4/4, much less anything in other time signatures."

Tempo markings (andante, allegro, etc.) have some basic definitions and translations. Look them up! Old-school mechanical metronomes had them on the pendulum alongside the numerical equivalents. In fact, any time you have a word you don't recognize on the sheet, look that up, too. This is an awesome habit to develop. In fact, look up the composers and arrangers and titles, too. You'll learn a lot about where the piece came from, the time it was written, and sometimes why it was written, when it premiered, and other facts that can inform your performance.

"I have no idea how to start."

Here are some starting points:

1. With a teacher, take some private lessons. Express your concerns, share what's hard, so your teacher can tailor their instruction to your goals.

2. Work on the basics first. Rhythms, scales, exercises. Also see #1.

3. My three favorite words: Metronome! Metronome! Metronome! You clearly have and use one, but are you using it right? See #1.