Sarah* moved across the country to a new town in her late 30s after fleeing a decade-long abusive marriage. Struggling to make friends and fill her lonely evenings, she joined a church choir - something that quickly became her only source of joy.

Her new GP had diagnosed her with depression and she started to see a female therapist. But when she failed to see noticeable improvement, she decided to take up the offer of help from her charismatic choirmaster, who told her he was a qualified counsellor.

"I had no understanding of boundaries in the therapeutic process, procedures, vulnerability, all those things. He seemed a decent person, highly respected in the church," she said.

Related: Psychotherapists in England must be regulated, experts say, after abuse claims rise

After three months of counselling sessions that she now alleges were a grooming period, they started having sex, and she claims he used the information she had shared about her vulnerabilities to make her progressively more reliant on him. "Everything was about power and control and secrecy. It became very abusive. It went on for a while, over which period my health deteriorated rapidly because of his gaslighting," she said.

"I just got more and more depressed and non-functioning. I had no control, it was like I was not really understanding the situation I was in. When I started to question anything he became very aggressive and controlling, telling me I couldn't go to choir, excluding me from things, which is something he knew I was sensitive about after my abusive marriage."

Sarah was referred to a psychiatrist after the abuse began, who signed her off work for three months. "[He] was really concerned that I wasn't responding to the medications he was prescribing. [He asked me:] 'What is going on in your life?' And it opened the floodgates. He made me aware it was professional abuse."

Sarah's story is an extreme example of bad and abusive therapy, but it illustrates how the current regulatory system makes it easier for rogue therapists to continue practising, and more difficult for victims to seek recourse in the event of misconduct.

Sarah claims she was not the first vulnerable woman to be abused by her counsellor. As the truth "started to dawn on me from this murk of depression", she began to suspect he was in relationships with other members of the choir.

She also learned that he had previously been employed as a priest in another town . One day, she visited the church where he had had been employed. "Just by serendipity I bumped into the current vicar and as soon as I mentioned the name he dropped his bicycle, went white and told me to go straight to the police. He told me that what his predecessor had done there had involved four female parishioners and had been tantamount to rape."

For Sarah there seemed to be no recourse - the church investigation went nowhere, and because the counsellor was not accredited there was no professional association to turn to for support. She thinks that had therapy been statutorily regulated she might have been more aware of how to verify her therapist's credentials, and it would have made it easier to file a complaint.

"You feel so foolish in retrospect, but I didn't understand the process and how you should select a therapist. Because I was at such a low ebb, I was completely vulnerable. I'd never been in that kind of situation before."