Blackmailed For Sex

I'm being blackmailed for sex. Or maybe there is a better term for it, I don't know. A year ago I (male in my 30s) started a casual FWB with a woman (in her 30s).
Soon after she wanted an actual relationship and I said that's not what I was looking for since I just got divorced and that I’d like to just be friends. She agreed and I thought that would be the end of that and we could continue hanging out platonically. She kept trying to make sex happen and I told her again that I just wanted to be friends. She started throwing tantrums and threatening suicide. I gave in and had sex with her again which is probably the worst thing I could have done.
She became even more attached and when I reiterated I just wanted to be friends, she threatened to tell police that I assaulted her. I was at a loss of what to do. I should have tried seeking legal advice then but I was scared. I gave in and for the last year I have been having sex with her when she asks me to come over. It's been tearing me up inside and I don't know what to do. I have been trying to gently extricate myself from the situation but anytime she senses me “pulling away” she starts acting erratically and alluding to her previous threat.
I am worried no one will believe me. She has been careful about threatening outright since the first time so the only thing I have are some texts where I try to say I'd like to just be friends and she begs me not to shut her out and to keep having sex with her. Is that enough to prove I haven't sexually assaulted her if she decides to follow through on her threat? I feel like I need more to prove it's not true but I don't know how to get it since she's been careful not to say it since the first time. Any advice would be appreciated, thank you. I feel stuck, scared, and like my life is in limbo right now.
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