H-B, I understand your point, but I'm pretty sure you're confusing this with an alleged sex crime that has had a delayed reporting with all evidence washed away. In a case like that, there's no hope of collecting any evidence, for example, the Bill Cosby case where some of the complaints come from 40 years ago.

The OP's situation is completely different.

This is blackmail, with the culprit demanding payment in sex rather than money. Because the threats are on-going, evidence of the culprit making a new demand and threat could be collected.

If the OP has any qualms about going to police or about the process that would be followed, I urge her to seek the advice of an attorney. Advice can also be sought through the rape, abuse and incest national network (RAINN.) This is definitely a case of sexual abuse.

https://www.rainn.org/

They have telephone and on-line hotlines with people who are experienced in helping traumatized, embarrassed and ashamed victims who don't know what to do. I strongly urge everyone to stand up against sexual abuse and exploitation, which, you know ... this is not theoretical on my part.