The day I met my rapist, a regular in the bar I worked in, I was told he was "a bit sleazy, but harmless". He raped me on a Saturday morning in September. It did not immediately hit me that I'd been raped. I wasn't dragged off the street. It wasn't violent. A group of us went to my flat after a party and the last thing I remember was sitting in my living room with other people. Then I woke up on my bed – with him on top of me.

This is not an account of how I was raped. This is an account of trying to prosecute a rapist. My actions after the event matter because they were examined in excruciating detail by the police and Crown Prosecution Service (CPS) and they are the reason that my rapist is free today.

I did what I now know to be pretty exceptional – I didn't hesitate to call the police. But as I waited for them I started to doubt myself. I had been drunk, I had taken drugs, I have a history of sexual promiscuity and mental illness and my memory was very patchy – a conviction seemed impossible. However, when they arrived the police tried to convince me it could happen.

They took a mouth swab and a urine sample and the next day – after a long significant shower – I went for a full examination, made bearable only by the amazing staff at St Mary's Sexual Assault Referral Centre. There were immediate decisions to make – notably whether to take anti-HIV medication, which has awful side effects. We had to dissect the likelihood of my attacker being HIV-positive. I took the medication and vomited daily for a month, most days violently and repeatedly. After vomiting there is no respite: you have to retake the pills until you can hold them down.

I quit my job the day after the assault, disgusted and terrified at the idea of seeing him, but talked to a colleague who was with me that night. Realising that I was unconscious, she had cleared all the people out my flat. He must have hidden and returned. I realised these were the actions of a predatory rapist and I felt I had no choice but to try to protect others from him. It was then I decided to prosecute.

At this point the police made one of several inconceivable mistakes. They had said that the swabs and bed sheets would be kept for several years. However, it transpired they had already destroyed all the forensic evidence – and blamed my initial hesitation.

It is easy to vilify the police. Please don't. They made mistakes, but they always treated me with as much respect as possible within the confines of their instructions – for me the villains of the story are the CPS.

The first interview was gruelling and intrusive; however it paled in comparison to the second, some months later. I was grilled like a suspect. I was disillusioned with the process and scared something I said would stop me getting my chance to look him, and a jury, in the eye and say: "This is what this man did. Do not let him do it again." Terrified, I edited my answers – and as a result I never got that chance.

They kept asking me if I had had sexual relations with anyone else there. Over the eight-month period and innumerable questions, I withheld one fragment of information from the police. A piece of information that had no bearing on the rape, or my rapist. I had been told I had gone into my bedroom earlier that evening with a different man, whom I had been kissing. I was so scared I would be judged because of this as promiscuous and unreliable. Of course, in this day and age promiscuity should have absolutely no impact on the perception of rape, but we all know it does. I didn't remember the incident, it wasn't my testimony, so I convinced myself I didn't need to tell them. Why should it matter? How was something I had done of my own volition relevant to my being raped? But the stifling level of scrutiny I was under made me feel sure it would matter, so I said nothing.

A month later, I got the call. They knew I had lied, which made me an "unreliable witness". The CPS had decided not to prosecute. They said "there was not a realistic prospect of conviction" as the jury wouldn't believe me. Why wasn't I given the opportunity to explain my actions, and convince them that I was raped?

Eight months of hell led to that phone call. I made a rash decision under pressure because of the stereotypes forced upon women ingrained in our legal system. A decision framed from a society obsessed with blaming women for making themselves "vulnerable" to rape, rather than targeting the rapists. One wrong decision led to a rapist walking free. I cannot forgive the CPS for this. I cannot forgive them for putting me in a position where I can't forgive myself. He doesn't even have to stand in a dock and – according to the police and the CPS – that's my fault.

With hindsight, would I encourage another rape victim to go to the police, to face months of interviews, examinations and implicit accusations – not from a jury but from the police and the omnipotent Crown Prosecution Service? I don't know that I could.

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