When Ernestine Jackson, who currently is singing in "Some Enchanted Evening" at the Terrace Theater, was a teen-ager in Corpus Christi, Tex., she thought that her dream of going to New York to be a "great concert artist" was an impossible one.

But then, in true fairy godmother fashion, the wealthy family of a friend she met in church offered to sponsor her for a year, loaning her whatever it would cost.

"They told me to make up a list of my expenses and said pay it back in 10 years at 2 percent interest," she said. "I came up with the figure of $5,000. I couldn't believe it."

So off she went. She enrolled in a summer program at the Juilliard School of Music and got a part-time job at Chock Full O' Nuts. That was in 1965, when New York was not quite as expensive as it is now, and she found she could manage quite well.

Jackson is a svelte soprano who gets called when "an actress who can sing" is needed, she said. She joined the five-person company of the Rodgers and Hammerstein revue in January while it was still at the St. Regis in New York. At that point the show was 45 minutes long and the company did 12 shows a week; the length was doubled for its transfer to the theater here and subsequent engagements this summer. (It plays here through Saturday.)

Jackson's days as a struggling young singer in New York (Chock Full O'Nuts gave way to a coveted job as a tour guide at the United Nations) didn't last long. First she got into the chorus of "Show Boat," and then into the Pearl Bailey version of "Hello, Dolly!" Since 1967 she has been able to support herself in the theater.

"People told me I shouldn't take chorus jobs because I'd never get a lead, but I don't believe in that," Jackson said. "I defy that. You have to get a rhythm going in this business. To me that's the rhythm of life."

She was able to pay back her Texas sponsors ahead of schedule when she played the young mother in the musical version of "Raisin in the Sun," a role not unlike the parts she has been cast for in commercials for Bufferin, Lysol Spray, Bounty paper towels, Geritol, Open Pit Barbecue Sauce and others. "I am always the mommy," she said.

Her original sponsors are not the least disappointed that she didn't become a concert artist, she said, nor is she.

"I don't enjoy the classics enough to spend a career on them," said Jackson, who is looking ahead to more musicals, a nightclub act, possibly a record, and a film.