A Waltham outpatient clinic that serves people struggling with severe mental illnesses is slashing its outpatient operations this month, leaving 350 patients to find new care providers and 11 clinicians to find new jobs even as other facilities face the same fate.

“This has been a very difficult decision for our agency,’’ said Ellen Attaliades, chief executive of the Edinburgh Center. “The rate of reimbursement that we receive from the insurance companies doesn’t fully support the cost of running the clinic.’’

Attaliades and others in the mental health field say the downsizing is symptomatic of a larger trend that’s been happening for years.

“I’ve been practicing since 1992. The issue of outpatient rates has been an issue the entire time,’’ said Marie Hobart, president of the Massachusetts Psychiatric Society and medical director for Community Healthlink Inc. in Worcester. “In the past five to 10 years when you look at the rate of inflation and then look at the rates for Medicaid and Medicare in particular, they just have not kept pace with the cost of doing business.’’

The fallout, she said, is that people are finding it increasingly difficult to get treatment for severe and persistent mental health issues, such as schizophrenia, bipolar disorder, and borderline personality disorders, because providers can’t afford to take on the clients.

Hobart said the Massachusetts Psychiatric Society has tried lobbying for mental health funding at the Legislature and governor’s office. “In recent years, it’s fallen on deaf ears,’’ she said.

Attaliades said that both private and public insurers are to blame for dwindling provider payments. “It’s all the insurances. It’s MassHealth and all of the managed-care entities,’’ she said.

A spokeswoman for the state’s Executive Office of Health and Human Services said that given the weak economic climate and the resulting state budget cuts, the agency sees the fact that reimbursement rates haven’t fallen as a positive development. Jennifer Kritz said reimbursement rates have remained flat for about five years.

But at the Edinburgh Center, this has translated to yearly deficits of about $400,000 for the past few years, said Attaliades.

She said that the cost of providing an hour of treatment ranges from $106 to $128. The Edinburgh Center gets back $66.27 an hour from MassHealth, which covers low-income residents, for psychiatric services and $62.71 an hour for counseling. A private contractor used by the state for mental health services reimburses at a slightly higher rate, but it’s still not enough, she said. “If our rates were increased about 40 percent, we’d be able to cover costs,’’ she said.