A Montgomery County jury last night found in favor of a Silver Spring woman who had accused her psychiatrist of malpractice for having an affair with her, but slashed by three-quarters the damages awarded her by a state health claims panel.

The jury of nine women and three men, after sitting through an eight-day trial and deliberating nearly six hours, found Dr. Gerald M. Fink, 53, guilty of medical malpractice for his affair with Frances M. Dawson, 44, a former patient.

But the jury, in what the foreman described as "a very, very agonizing decision," reduced a $274,946 compensatory damage award made to Dawson last summer by a state arbitration panel to $90,000. It awarded no punitive damages; the panel had awarded an additional $100,000 in punitive damages.

Reached by phone minutes after the midnight verdict, Dawson said, "though it hurts inside, at least I can hold my head up. I didn't have a settlement with the insurance company. At least I might have done something for the next woman" whose psychiatrist has an affair with her.

"I would have hoped for more money for my current therapy. Would I have hoped for Dr. Fink not to be found guilty? No," she said.

Fink, who looked down, showing no emotion when the verdict was read, said he was "grateful for the diminishing of the compensatory damages to 25 percent. I'm grateful there were no punitive damages. I'm also disappointed that they had any verdict against me." Fink's lawyers added that they plan to appeal the verdict.

Henry E. Weil, Dawson's attorney, had urged the jurors to increase the health claims panel's award to $1.8 million, saying that amount would cover more than $45,000 in medical bills, assure that Dawson would be cared for and warn other psychiatrists not to get sexually involved with their patients.

Dawson testified that Fink seduced her away from her 18-year marriage during her therapy, maintained a sexual relationship with her for 2 1/2 years, which included their living together for a year, then broke up with her after she signed a paper releasing him from malpractice liability.

Fink, who has been a psychiatrist for 23 years and practices in Silver Spring, testified that he and Dawson did not make love until three weeks after her therapy ended in September 1980.

Fink testified that he discussed Dawson's romantic feelings toward him with her at 16 sessions during her year-long treatment but did not act on them until he felt she was well and had stopped treating her.

Then he fell in love with her, leaving his wife of 27 years for Dawson, he testified. While they were living together she became too demanding, threatening to blackmail him if he did not put her name on the deed to the Silver Spring house they shared, Fink testified, and he gradually stopped loving her.

Fink's lawyers, Steven R. Migdal and Kenneth West, used opening and closing arguments and lines of questioning to promote the theme that Dawson's lawsuit arose from a broken heart rather than from medical malpractice. Migdal portrayed Dawson as motivated by revenge, calling her "a woman scorned" who demanded too much from Fink and who "blew her last chance to get her dream man."

Dawson, who has been hospitalized four times since her affair with Fink ended in March 1983, testified that he "raped my mind," by introducing sexual activity into their therapy sessions six months after they began in September 1979.

Weil, Dawson's attorney, told the jurors that whether they believed that sexual relations started between Dawson and Fink during therapy session or three weeks after the therapy ended, "it makes absolutely no difference . . . Dr Fink should have known better."