The St. Elsewhere star, 93, revealed how she and the Boy Meets World actor, 95, struggled as a couple after tying the knot in 1951.
"I guess it was a little bit of an open marriage at first, but that was very painful," she told Fox News Digital while promoting her new memoir Middle of the Rainbow. "That didn't work well. And it was a time when people were doing that. It was at a time in New York when there was a lot of sex and a lot of people doing all kinds of things, you know – very free."
"But I don't know if there was a lack of commitment a little bit, and that's not good," she continued. "So there was a lot of pain connected with any transgression, with any extramarital thing."
Ron Galella, Ltd./Ron Galella Collection
In her new memoir, Bartlett Daniels revealed "she never felt guilty" about extramarital affairs "because I never felt tied to fidelity, and neither did Bill," per the outlet.
She admitted to having "an affair that lasted a few months" with another actor in 1959 but her husband's affair with a New York-based producer in the early 1970s ultimately left her "devastated" and changed her outlook on their relationship.
At that point, she said she "could no longer tolerate any kind of open marriage."
The two eventually overcame their relationship difficulties — something Bartlett Daniels credited to their ability to "grow up together" and embrace their evolving marriage over the last seven decades.
"Life is all about solving problems," she said, per Fox News Digital. "We had a very good relationship, [but] Bill was an angry young man, a very angry young man. And that was tough… Bill always said, 'We had three different marriages. We've been together for so long, more than 70 years – a long time. I'm 93, and I met Bill when I was 18, so we had to grow up together."
She added, "Bill and I have moved forward day-by-day and eventually, the days added up. We've been happy together and sad together, and somehow stayed together for seven decades."
Bill Daniels Instagram
Reflecting on those early difficulties in her marriage, Bartlett Daniels told the outlet why she felt they were necessary to experience.
"It was something we had to go through because we never went through it," she said. "When we got together, I was 18. Bill was my first boyfriend… We just had to go through all that and still, we loved each other very much and always have."
"[We] have always been there for each other," she added. "That's what matters - if you're there for the person and help [them] along in a relationship, [have] respect for them and what they're doing and being there for them… [You have to] be together on the other side."
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In March, Daniels opened up about their relationship, revealing the secret to their long-lasting marriage and their "very content" life together at their home in Southern California.
"At first, there's a lot of struggle and there's a lot of uncertainty of whether you're in the right business at all when you can't get a job and you can be difficult — difficult to live with because of all this uncertainty," Daniels told Forbes. "What you really have to do is learn to respect the other person and their feelings and try to be as easy (laughs) to live with as you possibly can."
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"So much happens and you live — the good things, you hurt each other," Bartlett Daniels added at the time. "We've hurt each other but you recover and you grow and you change and you adapt. You have to be able to adapt to the other person. You have to think of them first. It doesn't happen overnight. It took us years to get to a real companionship marriage."