Her anxiety may be stopping her from starting...if that makes sense. Rabbit in headlight and all that.
Nagging won't help.
Helping her to organise might. My DD is Yr 10. To keep on top of things she, for a variety of reasons, still needs support. So each day she writes up on the fridge homework/due date. Separate column for tests.
We sit together (only 5 mins) and agree what's going to get done when. Things often don't go to plan, but she's got a plan and that makes her feel in control rather than overwhelmed. She updates the list at the end of the evening.
In year 7/8 we helped her devise revision cards. Now she's confident enough to do them herself. She created the cards for a topic, then DH or I reviewed the text book/work book to check all key items on revision cards and question if we thought something missing.
Before a test, we 'test' her on the revision cards she's done so she goes in confident that she's on top of the topic. Often she's in the kitchen when I'm cooking and I'm flipping through cards asking questions. It's not like University Challenge, more a joint review.
Our mantra is 'you're part of a team, not alone in this'. In any work environment people help their colleagues if needed, this is no different. Offer to sit with her and look at websites for revision, offer to help her devise revision plans, offer to talk about different revision methods. Active help, not nagging is your best bet.