THERE'S A LILT to her voice these days that hasn't been there in a long time. She can smile again, and it's not a harsh, self-pitying smile. She talks about her husband and their separation frankly, but not cruelly. She still loves him; she still hopes for a reconciliation. She still doesn't know what happened to her marriage, which she says was a happy one, but she is no longer obsessed by it.

Theirs was an old-fashioned surburban family. She married young and for love and afterwards she stayed at home and raised their four daughters. For years, she ran a child care group in her home. Her husband held a series of important congressional jobs, working the requisite 10- and 12-hour days and weekends when Congress was in session. They had, she says now, 20 good years together. Then something happened. She calls it the mid-life crisis.

"His job always came first. His family was there, but his job was so important that I learned to handle it. . . . He'd never been a failure, but he felt himself failing in simple, little things. The thing that was constantly on his mind was his job, his career. The responsibility of his job, his family, everything was kind of coming at him.

"He became uncommunicative, irritable. Little things bothered him. He was short with everybody, especially with the children. I just couldn't handle him coming home at night. He didn't want to talk. Today, I still don't think he knows what happened."

Her marriage, she says, became "total hell. He didn't want to have anything to do with me." The tense situation lasted four months, she said, and finally, she asked him to leave. "I was physically and emotionally drained. He was going to leave anyway."

He moved out last February. "For the next six months, I was on an all-time downer. I could no longer function at my job," taking care of the children in her play group. "I couldn't sleep. I lost weight. I couldn't eat. I was a mess. . . . I used to wake up and start shaking. He has no idea what I've been through. Nobody knows it unless they've been through it."

She watched her marriage mysteriously fall apart, and she awakened each day wondering what she had done wrong and finding no answers. The more she questioned her husband, the further she drove him away. She stopped. She entered therapy.

And late last summer, in a very businesslike manner, she telephoned the mothers of the children in her play group and told them she was closing her business. She'd taken brush-up courses in typing and shorthand and in her third interview she got a job as an administrative assistant to a man who didn't worry too much about the blank spots in her resume. He was an executive who led his corporation in hiring blacks and women managers. "He knew I was separated, but we never discussed it."

"I'm out there with people now. I'm not home with little kids. I still hurt a lot, but I'm not bitter. I like myself a lot better. I have my own identity. I'm doing a good job. I know I'm important. I was nervous, wondering if I could do a good job, wondering if I could handle that and my home life with those children, and wondering if the kids are going to have parties when I'm gone.

"I found out going back to work that I'm still attractive, that a lot of women envy my figure, which tickles me. I feel real good about myself."

She made changes. She traded in the jeans she wore at home for a working wardrobe. She cut her very long hair very short. "I needed a different look. It did a lot for me." There were other changes she didn't make. "I've had my chances to go skiing. I've accepted none of it."

She says her husband has continued to support the family financially and visits and telephones regularly. He spent the holidays at home. It is clear when she talks that she is still in love with him, but no longer in a kind of desperate, miserable way. "We used to have a really good time together. The thing I miss the most about all of this is not the sex part, but communication. We discussed everything. We dicussed ourselves, and how we felt about each other. We've been through a lot together."

She is 41, her husband is 43. "At my age, the aloneness scares me, but I still love him. I love him a lot. I can't say anything bad about him. . . . I don't ask him any questions. He's the best friend I ever had, but it's really hard when you can't talk to your best friend anymore. . . . He's a super guy. He really is. Or I wouldn't still be hanging on.

"I don't know what's going to happen. I'm preparing myself for either way. I have four kids who are uppermost in my mind at this point, and they're uppermost in his, I'm sure."

She says her job and the knowledge that her children are counting on her pulled her through, and even if she and her husband reconcile, she would continue working outside the home. "I think I have a future where I'm working."

A year ago, when she looked into the future, she saw herself and her husband retired to their mountain cabin, "enjoying our grandchildren together, sharing, catching up on all the time we missed together while we were busy making it."

Now, she thinks of a future in which she works, in which she hopes that she and her husband will have enough money to educate their children. "I think of happiness again. "But," she says, "I'm scared, too. I think of aloneness, mostly. . . . I don't know. I'm living one day at a time, at this point. And it's a crummy way to live."

But then she says something else she would not have said a year ago. "I had 20 really good years. I'm really fortunate. I'm going to make it one way or another. I know that."